Apple Mail on macOS repeatedly receives HTTP 429 from Exchange Online EWS after account re-add

Question

Apple Mail on macOS repeatedly receives HTTP 429 from Exchange Online EWS after account re-add

Alex 0

I’m having an Exchange Online synchronization issue with Apple Mail on macOS. The account worked normally in Apple Mail for approximately a year before suddenly stopping synchronization today.

Other access to the mailbox continues to work normally:

Outlook on the web works.
Mail on my iPhone works.
Apple Calendar on the same Mac continues to work with the Exchange account.
Other email accounts in Apple Mail work normally.

As part of troubleshooting, I removed the Exchange account from macOS and re-added it using the normal Microsoft 365 OAuth authentication flow. The account authenticated successfully, but Apple Mail still does not reliably synchronize.

Apple Mail intermittently displays errors such as:

“Unable to connect to server outlook.office365.com”
MCMailErrorDomain error 1029
“Connections to host (null) on the default ports failed”

However, macOS Console logs indicate that the underlying connection to Exchange Online is actually succeeding. During a manual synchronization, exchangesyncd:

successfully resolves and connects to Exchange Online
successfully completes TLS
receives multiple HTTP 200 responses
begins an EWS Inbox synchronization
receives a non-final batch containing 512 created items
subsequently begins receiving repeated HTTP 429 responses

The logs explicitly show:

received response, status 429

and:

SOAPWebServicesErrorDomain ... SOAPErrorHTTPStatusKey=429

There were approximately 30 HTTP 429 responses during the captured synchronization attempt. After the 429 responses, Apple Mail repeatedly reconnects and/or attempts Autodiscover, even though the underlying network and TLS connections continue to succeed.

I searched the captured logs for Retry-After, BackOffMilliseconds, and ErrorServerBusy, but none were present.

Since this started after approximately a year of normal operation, and the account has now been re-added and appears to be performing a substantial initial EWS synchronization, I’m trying to determine the safest way to recover without deleting the account or resetting its synchronization state again.

My questions are:

What does repeated HTTP 429 during SyncFolderItems/subsequent EWS synchronization indicate in Exchange Online when no Retry-After or BackOffMilliseconds value is returned?
Is there a recommended recovery procedure for Apple Mail when an initial Exchange Online synchronization becomes EWS-throttled?
Should Apple Mail be left closed for a period before allowing synchronization to resume, and if so, is there a documented backoff period?
Is there any way to preserve the current EWS synchronization state rather than removing/re-adding or rebuilding the account again?
Is there a known current issue involving Apple Mail/macOS exchangesyncd and Exchange Online that could produce this behavior?

I would prefer not to remove/re-add the account again or rebuild the mailbox because that may trigger another full synchronization. Any guidance on interpreting the 429 responses or safely allowing the existing synchronization to recover would be appreciated.

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1 answer

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Answer 1

Ryan-N 15,555 Microsoft External Staff Moderator

Hi Alex,

Welcome to the Microsoft Q&A forum.

Based on the information you provided, it is highly likely that you are experiencing EWS throttling (rate limiting) in Exchange Online because your mailbox contains a large amount of data.

This is a mechanism implemented by Microsoft to protect system resources when an application attempts to synchronize a large volume of data within a short period of time, helping to ensure service stability.

In this situation, I do not recommend removing and re-adding the account, as doing so may create a loop that forces all data to be synchronized again from the beginning.

At this time, I have not found any information indicating that this is a widespread issue or a global service outage.

In the meantime, I recommend that you close the Apple Mail application, wait approximately 24 hours, and then check again.

If the issue continues to occur after 24 hours, please reply to this post. I will continue to assist you.

If the answer is helpful, please click "Yes" and kindly upvote it.

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Devin 0 Reputation points

2026-09-10T13:58:41.6666667+00:00

I suspect it's a widespread issue - I'm having the same problem.

Please keep me posted as to the solution.

Thanks!
Productions Déferlantes 0 Reputation points

2026-09-10T14:07:27.63+00:00

I'm having the same issue on 5 different M365 accounts from 5 different tenant in my app Mail. Some of them contain only 5 emails. They all stopped working this morning. Same result from my office computer and personnal iMac computer both on Tahoe 26.
Anthony Pizzimenti 0 Reputation points

2026-09-10T14:46:16.6666667+00:00

I'm having the same problem as well, and have a mailbox rule that deletes any message more than two years old.
Timothy Lethbridge 0 Reputation points

2026-09-10T15:01:12.34+00:00

I am having the same problem; I see on DownDetector that there are complaints too, so this must be a widespread problem. Telling us to wait 24 hours (during which our workflow is disrupted) is a serious problem for many people and is not tolerable.