Outlook task pane and buttons
Read and compose buttons that open a task pane with your system's data beside the message.
The Problem
Your Outlook Add-in Works in Classic Outlook and Nowhere Else
It shows up when your company starts moving people to new Outlook for Windows or Outlook on the web. COM and VSTO add-ins don't load there, so a daily workflow quietly vanishes.
01 · New Outlook
Your COM add-in vanishes in new Outlook.
A web add-in on Office.js loads in new and classic Outlook.
02 · Mac and web
Mac and web users never had the add-in.
The same add-in runs on Mac and in Outlook on the web.
03 · Tokens
Calls fail since legacy Exchange tokens were turned off.
Sign-in moves to nested app authentication with MSAL.
04 · Rollout
Every laptop needs its own installer.
IT deploys once from Integrated apps in the admin center.
Your team files client emails into your CRM with a button someone built in VSTO years ago. Then a few people switch to new Outlook, and the button is gone. Mac users never had it. A compliance check that warned before sending stops working, because the Exchange tokens it used were turned off. Finance still runs a VBA macro only one person understands. IT wants one add-in it can deploy to everyone.
We start with what your current add-in or macro really does, then rebuild it as a web add-in on Office.js. It loads in new and classic Outlook, on the web, and on Mac. Sign-in moves to nested app authentication, send-time checks become Smart Alerts, and IT deploys it once. The workflow comes back for everyone, not just one laptop.
One add-in, every Outlook
Web add-ins load in new and classic Outlook, in Outlook on the web, and on Mac.
Checks before Send
Smart Alerts warn about or block a message that breaks a rule before it leaves.
Sign-in that still works
Nested app authentication replaces the legacy Exchange tokens Microsoft has turned off.
Our Outlook Add-in Development Process
From Add-in Review to a Deployed Add-in in Five Clear Steps
You approve every button, permission, and manifest choice before code is written, and we test in each Outlook your people use before IT deploys it.
We list what your current COM add-in, VSTO add-in, or macro does, feature by feature, and mark which ones Office.js supports. You get a feature plan and a fixed quote. If a Marketplace add-in already covers it, we'll tell you before we quote.
What's Included
What You Get With Microsoft Outlook Add-in Development
Outlook buttons and task panes, send-time checks, Excel and Word add-ins where they help, modern sign-in, and a deployment your IT team runs once for everyone.
Compare Options
DeveloperLook vs Keeping the COM Add-in vs Freelancer
Keeping a COM add-in costs nothing today and keeps Windows-only features, and a freelancer may be cheaper for one button. This is how the options compare as new Outlook spreads.
| Feature | DeveloperLookBest fit | Keep COM add-in | Freelancer |
|---|---|---|---|
| Works in new Outlook | Varies by person | ||
| Mac and web | Varies by person | ||
| Upfront cost | Fixed quote per feature | Already paid for | Hourly |
| Windows-only features | Office.js features only | Full Windows access | Office.js features only |
| Sign-in | Nested app authentication | Older desktop sign-in | Often legacy tokens |
| Deployment | Once from Integrated apps | Installer per machine | Varies by person |
| Testing | Every Outlook your team uses | Already in use | Often one client only |
| Code ownership | Your code and manifest | Yours, if you have the source | Depends on the contract |
DeveloperLook
Keep COM add-in
- Works in new Outlook
- DeveloperLook: Keep COM add-in:
- Mac and web
- DeveloperLook: Keep COM add-in:
- Upfront cost
- DeveloperLook: Fixed quote per featureKeep COM add-in: Already paid for
- Windows-only features
- DeveloperLook: Office.js features onlyKeep COM add-in: Full Windows access
- Sign-in
- DeveloperLook: Nested app authenticationKeep COM add-in: Older desktop sign-in
- Deployment
- DeveloperLook: Once from Integrated appsKeep COM add-in: Installer per machine
- Testing
- DeveloperLook: Every Outlook your team usesKeep COM add-in: Already in use
- Code ownership
- DeveloperLook: Your code and manifestKeep COM add-in: Yours, if you have the source
Projects We've Built
Finance, Investment, and Insurance Projects We Have Built
Finance, investment, consulting, and insurance projects we designed and built. None is an Office add-in, but each serves teams who run client work through email and spreadsheets.
Armada AG Fiduciary and Consulting Services Website
- Finance
- Business
- Website
Moneywish Finance App Design & Development
- Finance
- Dashboard
- Mobile App
Envitis Investment Website Design & Development
- Finance
- Dashboard
- Website
EverTrust Insurance Web Design & Development
- Insurance
- Dashboard
- Website
What Our Clients Say
What Clients Say After Launch
Feedback from teams we have built websites, apps, and integrations for.
We had a pretty specific idea of what we wanted our website to become, but I wasn’t sure how easily we could explain all of it to a development team. There were quite a few moving parts, and we were working toward a deadline, so I expected a few things to get lost along the way. That never really happened. They understood what we were trying to achieve, kept us updated throughout the process, and got everything ready within the agreed timeline. The final site feels much easier to navigate, works beautifully on mobile, and is noticeably smoother for our customers. It was a huge relief seeing the original idea actually turn into the website we had in mind.
The mobile experience is probably my favourite part of the new site. We had spent a lot of time making sure the desktop version looked right, and the team kept pushing us to think about how everything would actually feel on a phone too. They were right. The navigation is clean, the interactions feel natural, and everything we’ve tested so far works exactly as it should. They also went through the links, forms, integrations, and other details more than once before launch, which I really appreciated. And even after going live, we could still reach out when something came up. The whole process felt friendly, organised, and surprisingly straightforward.
What I appreciate most is that the website is actually easy for us to manage now. We can update content ourselves instead of sending a developer a message every time we need to change something small. They also paid attention to the technical SEO side, which mattered to us because we didn’t want a new website to create problems for the traffic we already had. I’ve probably asked more questions than I should have since launch, but every question has been handled patiently and without any fuss. The team is easy to work with, they explain things clearly, and when we need something changed, they focus on getting it done rather than making the process complicated.
1,250+
Projects shipped
10+
Years of experience
Top 3%
Upwork Top Rated Plus
5 stars
Average review score
Want results like these?
Tell us what you are building. You get a reply within 24 hours on business days, then a written estimate.
Work Culture
Why Microsoft 365 Teams Trust Us With Their Outlook Add-in
An add-in reads mail and documents your people send to clients, so these habits decide how it treats their mailboxes, their data, and your IT team.
Inventory before any rebuild
We list every feature of your old add-in or macro before quoting. Nothing your team relies on gets dropped by surprise in the move.
Tested in every Outlook
Each build is sideloaded into new and classic Outlook, the web, and Mac. Bugs show up on our test mailboxes, not your partners' inboxes.
Straight talk about gaps
A few COM features have no web equivalent yet. We name them in the review, so you can plan around them before you pay for a rebuild.
Least mailbox permissions
The add-in asks only for the item and mailbox access its buttons use. Your IT team approves a shorter list, and client mail stays better protected.
One price per feature
Your quote prices each button, check, and task pane on its own line. A feature you add later gets its own quote before we build it.
Your manifest, your tenant
The code, the manifest, and the app registration live in your tenant and repository. You can change developers without redeploying from scratch.
Overview
Who handles Outlook add-in development for Microsoft 365 teams?
DeveloperLook handles Outlook add-in development for companies and software vendors on Microsoft 365. We build Outlook task panes, Smart Alerts, Excel and Word add-ins, and replacements for COM and VSTO add-ins that new Outlook won't load. Add-ins start from $700, take 2 to 10 weeks, and the code stays yours.
DEVELOPERLOOK
Project facts · Outlook Add-in Development
- Service
- Outlook add-in development (Outlook, Excel, and Word add-ins, COM and VSTO replacement)
- Typical timeline
- 2 to 4 weeks for one Outlook add-in, 5 to 10 weeks for suites or rebuilds
- Platforms
- Outlook on Windows (new and classic), Mac, web, and mobile, plus Excel and Word
- Built with
- Office.js, the unified or XML manifest, React, MSAL with nested app authentication
- Experience
- 10+ years of development experience and 1,250+ shipped projects
- Post-launch support
- 30, 60, or 90 days, depending on plan
- Communication
- Calls in US, UK, and AU business hours
- Starting price
- From $700 USD, fixed scope after a free add-in review
Reviewed 22 Oct 2026 by Saimul Islam
Good fit
Companies moving to new Outlook
Your staff rely on a COM or VSTO add-in in classic Outlook, and it disappears the moment someone switches to new Outlook for Windows.
Teams filing email into a CRM
Your team copies client emails and attachments into your CRM or matter system by hand, one message at a time, all day.
Finance teams living in Excel
Your analysts depend on VBA macros or XLL functions, and you want Excel add-in development that also works on the web and Mac.
Software vendors on Microsoft 365
Your customers want your product beside their email, and you want Microsoft Office add-ins they can install from the Marketplace.
Agencies with Microsoft 365 clients
Your agency serves clients who run on Outlook and Excel and needs engineers who build and deploy Office add-ins under your name.
Not a fit
Your team uses Google Workspace
Your company reads email in Gmail and writes in Google Docs. Our Google Workspace Add-on Development page covers add-ons for that setup.
You want a chat app instead
Your team wants tools inside Teams chats and meetings, not beside an email. Our Microsoft Teams App Development page fits that work.
Windows-only hooks required
Your add-in depends on Windows-only features such as custom proofing tools. Web add-ins don't support those, so classic Outlook may stay your home for now.
A Marketplace add-in exists
An existing add-in already files email into your CRM or signs documents. Installing it costs less than building one, and we'll say so.
Not sure we're the right fit?
Book a free call or message us. We'll tell you honestly whether we're the right team.
Pricing
Transparent Starting Prices for Outlook Add-in Development
These are starting points in USD. Your final price depends on your features, Office apps, and legacy code to replace, and you get a fixed proposal after a free add-in review.
For teams that need one Outlook workflow back, such as filing email to a CRM.
$700one-time, USD
- Free add-in review and feature plan
- Read and compose task pane
- Connection to one system or API
- Sign-in with nested app authentication
- Integrated apps deployment
- 30 days of post-launch support
Recommended
For teams that need Outlook plus an Excel or Word add-in.
$2,200one-time, USD
- Everything in Outlook Add-in
- Excel or Word add-in in the same package
- Unified manifest where your clients support it
- Smart Alerts checks on Send
- Excel custom functions if needed
- 60 days of post-launch support
For companies replacing a legacy COM or VSTO add-in across Office apps.
$5,400one-time, USD
- Everything in Office Add-in Suite
- Legacy add-in rebuilt feature by feature
- Outlook, Excel, and Word coverage
- Event-based activation for background tasks
- Microsoft Marketplace submission if needed
- 90 days of post-launch support
For Agencies
White-Label Office Add-in Development for Agencies and Studios
You own the client relationship. We supply the add-in engineering: feature inventory, Office.js code, modern sign-in, and admin deployment. Your client sees your agency's name, and every file lands in their own tenant.
How the partnership works
White label: your client only ever sees your brand
- Your client
- You, the agency
- DeveloperLook
Share the client brief
Send the client's current add-in or macro and the Office apps they use.
- Client needs
- You share the details
- Details received
We scope and staff
Within days you get a feature plan, a fixed price, and the engineer's name.
- Scope and plan
- Proposal to resell
- Your quoteYOUR LOGO
Build and hand off
We build, test in every Outlook, deploy, and hand over under your brand.
- Done by our team
- Your brand on it
- Handed to your clientDone
Partner with DeveloperLookDelivered under your brand
Stay client-facing
You run the client calls while we build, test, and deploy.
Predictable delivery
A fixed scope per feature, with dates you can promise your client.
Senior add-in engineers
Engineers who know Office.js, manifests, Smart Alerts, and nested app authentication.
Rollout notes included
Deployment steps for each client's Microsoft 365 admin, in plain language.
The Right Technology for Your Project
Technology Stack
The Office apps, add-in tools, front-end stack, and identity services we use to build add-ins that load wherever your people read mail.
- Microsoft Outlook
- Microsoft Excel
- Microsoft Word
- Microsoft PowerPoint
- Office.js
- Unified manifest
- Smart Alerts
- Yeoman generator
- Agents Toolkit
React
TypeScript
Webpack- Fluent UI
- Microsoft Entra ID
- MSAL
- Microsoft Graph
Microsoft Azure
FAQ
Frequently Asked Questions
Straight answers on add-in cost, timelines, installing and sideloading, new Outlook, sign-in, and Excel functions.
Saimul IslamFounder & CEO, DeveloperLookStill have questions?
Send a message or book a call with the team.
An Outlook add-in from us starts at $700 for a read and compose task pane connected to one system, deployed by your admin. Adding an Excel or Word add-in and Smart Alerts is $2,200, and full COM or VSTO replacements start at $5,400. You get a fixed quote after a free add-in review.
A single Outlook add-in takes 2 to 4 weeks, including tests in every Outlook client. Suites and COM rebuilds take 5 to 10 weeks. After deployment from Integrated apps, the add-in can take 24 to 72 hours to reach users, and Marketplace review typically takes four to six weeks.
In Excel, Word, or PowerPoint, select Home, then Add-ins, then More Add-ins, and add one from the Store tab. In new Outlook and Outlook on the web, use More apps, then Add apps. Most companies skip this step: admins deploy Office add-ins to everyone from Integrated apps.
You sideload it. Open aka.ms/olksideload, go to My add-ins, then Custom Addins, choose Add a custom add-in, and pick Add from File with the manifest. The Add from URL option is gone. Classic Outlook can take up to 24 hours to show a sideloaded add-in.
New Outlook for Windows runs only web add-ins built on Office.js, so COM and VSTO add-ins don't load there. They still work in classic Outlook, which Microsoft supports until at least 2029. Microsoft's guidance is to migrate them to web add-ins before your people switch.
Microsoft turned off legacy Exchange user identity and callback tokens across all Microsoft 365 tenants, so add-ins that rely on them now fail with errors. The replacement is nested app authentication with MSAL. Exchange on-premises isn't affected, but most add-ins we see need the update.
Yes, with Smart Alerts. The add-in runs when someone selects Send and can warn them, ask for a fix, or block the message. It works in Outlook on the web, new Outlook, classic Outlook 2206 or later, and Mac, but not on mobile. Add-ins that use block mode can't be listed on the Marketplace.
An Excel add-in developer rewrites them as JavaScript custom functions that appear in Excel like built-in ones. They run on Windows, Mac, and the web, but not on iPad or Office 2021 and earlier. For heavy Windows math, the manifest can point to an XLL that Windows uses instead.
Use the unified manifest when your people are on Microsoft 365 for Windows and the web: it's generally available for Outlook, Word, Excel, and PowerPoint. Keep the XML manifest when you need Outlook on Mac or mobile, older perpetual Office, or Exchange on-premises. Before you develop Office add-in features, we check which applies.
Hire Outlook add-in developers when the add-in must work with your own CRM, matter system, or rules, or replace a legacy add-in. Buy a Marketplace add-in for common jobs like e-signatures. With us, the code and manifest stay yours, and plans include 30 to 90 days of support.
Saimul IslamFounder & CEO, DeveloperLookStill have questions?
Send a message or book a call with the team.
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