A staff member opens the mail app they have used for years. New messages do not appear. Sent mail sits in the outbox. The shared calendar stops updating. Outlook on the web still works. The official Outlook app still works. The third-party app on the laptop or phone does not.
That is the kind of quiet break Microsoft is starting this month.
What is changing
Microsoft is turning off an older connection method called Exchange Web Services, usually shortened to EWS. Many non-Microsoft mail and calendar apps used EWS to reach a Microsoft 365 mailbox. Microsoft announced years ago that EWS would stop receiving new features. The cutoff is now on the calendar.
Phased disablement in Exchange Online starts in October 2026. Full retirement is set for April 1, 2027. Between those dates, access gets tighter. Some tenants see EWS blocked unless an admin has an allow list of specific apps. Apps that still depend on EWS can lose send, receive, or calendar sync with little warning. The mailbox itself is fine. The old doorway into it is closing.
Who is usually fine
People who already use Microsoft’s own tools are in the safer group:
- Outlook on the web at outlook.office.com
- The Outlook app for Windows, on a current build
- The new Outlook for Mac
- Outlook for iPhone and Android
Those clients talk to Microsoft 365 through the newer method, Microsoft Graph. No app switch is required for that group.
Who is more likely to feel it
The risk sits with third-party mail and calendar apps that still reach the mailbox through EWS. Names that keep showing up in university and vendor notices include older Apple Mail on some Mac versions, Spark, Thunderbird with certain add-ons, eM Client, Mailbird, Airmail, Canary Mail, Fantastical, and Edison Mail. Backup tools, CRM connectors, and helpdesk systems that read a mailbox through EWS can be in the same boat.
This list is not complete. An app can look modern and still use the old connection under the hood. The only reliable test is whether that app still syncs after the October changes, not the logo on the icon.
How to tell what staff are actually using
Do this in one sitting. You do not need to change anything yet.
- Ask each person which app they open for work mail. Write down the name and the device: office PC, home laptop, iPhone, Android phone, iPad.
- Look for anything that is not Outlook. Apple Mail, Thunderbird, Spark, and “the mail app that came with the phone” are the usual answers.
- On a phone, open the mail account settings and note whether the account type says Exchange, Microsoft 365, or something generic like IMAP. IMAP-only setups are a separate topic, but they will not get calendar and contact sync the way an Exchange-style app does.
- If you have Microsoft 365 admin access, sign in to the admin center and review active users. You are only confirming who has a mailbox. The app each person uses is still something you learn by asking.
- Check shared mailboxes and “info@” addresses the same way. Those often live in a third-party app on one person’s desktop and get forgotten.
A leftover app on one phone is enough to make invoices look late.
What to test this week
Pick one normal workday and send three short tests. Use a real message, not a blank one.
- From the official Outlook app or Outlook on the web, send a note to a Gmail or Yahoo address you control. Confirm it arrives and that a reply comes back into the Microsoft 365 inbox.
- From the third-party app a staff member prefers, send the same kind of note. Then reply from the outside address and see whether the reply lands in that app, not only in Outlook on the web.
- Add a test appointment on the shared calendar in the third-party app. Open Outlook on the web and see whether the appointment is there. Then create one in Outlook on the web and see whether the third-party app picks it up within a few minutes.
If send works and calendar sync does not, treat that as a fail. Calendar and contacts are often the first pieces to drop when EWS is restricted.
If a test fails, switch that person to Outlook on the web for the day so quotes and invoices still move. Then plan the app change. Do not keep retrying a broken connection and assume it will heal on its own.
Safer options
The simplest path is the official Outlook app or Outlook on the web. Both stay on the supported connection. Install Outlook, sign in with the work address, and send the three tests again.
If only a few people need the full Microsoft 365 mailbox and others just need a branded address with reliable phone sync, a KloudEmail or Hosted Exchange mailbox on the same domain can be the cleaner fit. KartHost already mixes mailbox types that way when a team does not need every seat on Microsoft 365. The point is a supported sync path, not a favorite icon.
A one-page version you can keep
- List every app that opens company mail, including phones.
- Circle anything that is not Outlook.
- Send and reply once from Outlook on the web.
- Send, reply, and calendar-test once from each third-party app.
- Move anyone who fails the test to Outlook on the web until the app is updated or replaced.
- Recheck after October 10, when allow-list rules tighten for many tenants.
- Plan to be off EWS completely before April 1, 2027, when Microsoft says the old connection ends for good.
This check is not a full mailbox rebuild. It is a way to find the one app that will go quiet before a customer notices. Related reading if you want the wider picture: Microsoft 365 Mailbox Storage Changes Small Businesses Should Understand, How to Prepare Your Business Email Before Fall Gets Busy, and Email Security Checklist for Small Businesses.
Busy season does not wait on a mail app. A short test this week shows who is still on the old doorway, while there is still time to switch.
