Outlook 365 for Mac sync loop

Question

Outlook 365 for Mac sync loop

carlan nanan 0

Hi

I am hoping to get some help with this.

I have 3 Apple Mac's with different version of MacOS. 1 with Ventura and 2 with Tahoe(1 of which is on the latest)

Outlook 365(legacy outlook). different versions. 1 macbook is on the latest version of outlook.

What's happening is. All 3 macbooks are continously syncing emails. They are all setup on IMAP.

They sync the mails and then continue once again when done.

So far, I have reset outlook completely, deleted the "Main Profile", uninstalled and re-installed, removed and re-installed from app store, updated the OS(which is why 1 macbook is on the latest OS.

I have already checked with my service providers as we are on hosted POP.

We have 2 different domains and both do the same. So I don't think it's service provider issue.

And none of my windows PC's do this.

I am at my wits end. I need some help please.

Kind regards

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Answer 1

carlan nanan 0

I have found a solution.
I deleted the "Main Profile" again for probably the 5th time.
I then downloaded a fresh copy of oultook from https://learn.microsoft.com/en-us/officeupdates/update-history-office-for-mac

After that I gave outlook full disk access under Privacy&Security settings.
Then after re-adding the mail account in outlook. I deleted everything in the drafts folder.

I have fixed 2 Apple Mac's so far. 1 iMac & 1 Macbook pro.

This seems to have fixed the issue of the sync loop.

Thank you so much Martin and Chloe for your input.

Chloe-V 7,650 Reputation points Microsoft External Staff Moderator

2026-04-26T06:53:50.3633333+00:00

Hi carlan nanan,

Thanks for sharing the solution—really appreciate you taking the time to post this.

Answer 2

Hi carlan nanan,

Welcome to Microsoft Q&A forum.

It’s completely understandable how frustrating it can be when Outlook on multiple Mac devices keeps showing “Syncing” continuously, even after resetting profiles and reinstalling the app several times.

With IMAP accounts in Legacy Outlook for Mac, this behavior is often related to advanced IMAP settings. You may want to check the following in order:

Go to Outlook > Settings > Accounts > select the IMAP account > Advanced. If the option “Use IMAP IDLE (if the server supports it)” is available, try turning it off temporarily and set “Sync all IMAP folders every x minutes” to 10–30 minutes to avoid constant update cycles.
In Advanced, try enabling “Download headers only” to reduce the amount of data being downloaded, then observe whether the sync loop decreases or stops.
Check the folder mapping (Sent/Drafts/Junk/Trash) to make sure it matches the actual folder structure on the mail server, as incorrect mapping can cause Outlook to repeatedly resync the same data.

Reference documentation for advanced IMAP options: Advanced IMAP account settings for Outlook for Mac

If the issue continues after trying the steps above, you can refer to “Fixes or workarounds for recent issues in Outlook for Mac” and use Help > Contact Support in Outlook to submit details to the support team.

Please note that this is a user-to-user support forum. Moderators, contributors including external Microsoft employees cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas only.

Thank you for your understanding.

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carlan nanan 0 Reputation points

2026-04-17T06:57:45.01+00:00

Hi Chloe

Thank you for your response.

I have tried these steps already. I have disabled the IMAP Idle & "Sync every x minutes".

I have enabled "Download Headers Only". And I have checked the folder mappings to "server".

What I am noticing is, Outlook does not complete the download.
For example. On the server there are 1148 items in the sent folder. and outlook has downloaded 1139 items. and then tries to download the other 9. But that process does not complete. It downloads 2 items and then it just stops. Then I have to click send/receive or go to inbox and back to sent for it to try syncing again.
Chloe-V 7,650 Reputation points Microsoft External Staff Moderator

2026-04-19T03:08:50.9866667+00:00

Hi carlan nanan,

Thank you for confirming you’ve already disabled IMAP IDLE/sync interval, enabled “Download headers only,” and verified the folder mapping. The fact that Outlook stalls on the last few messages (for example, 1139/1148 in Sent) and only continues after you click Send/Receive or switch folders usually indicates it’s getting stuck on one or two specific items. Please try signing in to the mailbox in a browser to confirm the folder shows the full item count and check whether the last few messages open/move normally.

If webmail/another IMAP app (or the new Outlook) works fine but the old Outlook still gets stuck, in that case please contact Microsoft Support for further assistance.

Go to Contact Us: https://support.microsoft.com/en-us/contactus

Click Show expanded list of products

Select Outlook > Get Home support

Type Outlook live chat in the search box, then select Get Help > Contact Support

If you can’t sign in to the affected account, sign in with a different Microsoft account to access support options

Under Products & Services, select Other Products > Outlook, then choose Chat with a support agent via web browser (if available)

Please note that this is a user-to-user support forum. Moderators and contributors (including external Microsoft employees) cannot directly intervene in Microsoft product features or access back-end systems. Our role is limited to providing technical guidance on reported issues, requests, or ideas only.

Thank you for your understanding.

Answer 3

Hi, Carlan,

Continuous IMAP syncing on multiple Macs — especially when Windows machines are fine — usually points to an Outlook for Mac–specific issue rather than a server or domain problem. You’ve already done most of the right troubleshooting steps, so here are the areas that typically cause this behavior:

1. Legacy Outlook for Mac has known IMAP sync issues

The “Legacy Outlook” client on macOS is well‑known for:

Re‑syncing the entire mailbox repeatedly

Getting stuck in a loop when folders or flags don’t match the server

Handling IMAP differently across macOS versions

If possible, switch one device to the New Outlook for Mac and test whether the issue stops. This is the quickest way to confirm whether the problem is client‑related.

2. IMAP folder mapping conflicts

On macOS, Outlook sometimes mismatches:

Sent

Drafts

Junk

Trash

If these don’t match the server’s folder structure, Outlook keeps re‑syncing. Check Outlook > Accounts > Advanced > Folders and make sure each folder is mapped to the correct server folder.

3. Large mailboxes or many subfolders

If the mailbox has:

Thousands of messages

Many nested folders

Large attachments

Outlook for Mac may continuously re‑index. Try temporarily disabling “Download full messages” to see if syncing stabilizes.

4. Corrupted local Outlook database

Even after reinstalling, Outlook may reuse cached data from macOS. Make sure you also remove:

~/Library/Group Containers/UBF8T346G9.Office/Outlook

This forces Outlook to rebuild everything from scratch.

5. Server‑side POP/IMAP hybrid setups

You mentioned the provider hosts POP accounts. If the server is doing POP → IMAP translation or has mixed protocols, Outlook for Mac is more sensitive to it than Windows Outlook.

Testing the same account in Apple Mail on one Mac can help confirm whether the issue is Outlook‑specific.

If you can share whether the issue happens on the New Outlook or only Legacy Outlook, I can help narrow it down further.

Hope this helps

Martin

I look forward to your update and am happy to continue working with you until the issue is resolved. If you find the answer helpful, please click "Accept Answer" and consider upvoting it. Otherwise, please keep me posted by clicking "Add comments" below instead of selecting Yes or No.

This response is drafted with the help of AI systems.

carlan nanan 0 Reputation points

2026-04-16T13:50:36.7366667+00:00

Hi Martin

Thank you for your reply.
I have done all the above. Number 4 especially has been done at least 6 times.

I just do not know what it could be.

New Outlook just says at the top "On My Computer data is currently being imported"

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Outlook 365 for Mac sync loop

2 additional answers

1. Legacy Outlook for Mac has known IMAP sync issues

2. IMAP folder mapping conflicts

3. Large mailboxes or many subfolders

4. Corrupted local Outlook database

5. Server‑side POP/IMAP hybrid setups

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