Outlook for Mac (Legacy Outlook) not syncing one email account after switching from New Outlook

Question

Outlook for Mac (Legacy Outlook) not syncing one email account after switching from New Outlook

Vinud Aslama 0

Hi,

I’m having a serious synchronization issue with Outlook for Mac (Legacy Outlook) on the Mac.

The problem started after he switched from the New Outlook interface to Legacy Outlook. Before switching, this particular email account was working normally.

Here are the details:

Mac: Apple M2 Pro
macOS: Tahoe 26.6.2
Outlook: 16.112.3 (26083020)
Outlook mode: Legacy Outlook
We have multiple email accounts in Outlook, but only one account is experiencing this issue.
The problematic account works normally through webmail and mobile, so the mailbox itself appears to be accessible.
The mailbox contains approximately 17,000+ email threads.
Outlook starts synchronizing the mailbox but takes an extremely long time.
At one point, synchronization reached approximately 12,000–15,000 emails, but then the process encountered problems/restarted.
A second Mac used by the same user is also experiencing problems with this account.

We were advised to delete or rebuild the local Outlook database/cache, but we have not done this because we cannot locate the correct Outlook local data folder and we don't want to accidentally lose locally stored email data.

We would like to know:

Could this be a Legacy Outlook-specific synchronization/database issue?
Is there a known issue with Outlook 16.112.3 on macOS Tahoe 26.6.2?
Is it possible to roll back to an earlier Outlook version to test whether the problem is related to the current build?
Where exactly does Legacy Outlook 16.112.3 store its local mailbox/database on macOS Tahoe?
Is there a safe way to rebuild the local Outlook database/cache without losing the existing mailbox data?

Any advice would be greatly appreciated. We would especially like to avoid deleting the Outlook profile/database until we know exactly what is stored locally and whether it can be safely rebuilt.

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1 answer

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Answer 1

Liora 1,720 Independent Advisor

Dear @Vinud Aslama,

I hope you’re having a good day.

Based on your description, I do not think the mailbox itself is the primary issue. The strongest indicators are:

The account works normally in webmail and on mobile.
The issue started after switching from New Outlook to Legacy Outlook.
The same mailbox shows similar behavior on a second Mac.
Other accounts in the same Outlook profile are unaffected.

Taken together, this points more toward a Legacy Outlook synchronization issue affecting this specific mailbox rather than a general mailbox access issue.

One important question before considering any cache or profile rebuild: Is the affected account a Microsoft 365 / Exchange Online mailbox or an IMAP account?

This is important because, for Exchange Online mailboxes, the local Outlook database is primarily a synchronized copy of data stored on the server. In that scenario, rebuilding the local profile is generally much lower risk than it would be for an account that contains local-only data.

Regarding the local Outlook data location, Legacy Outlook typically stores profile data under:

~/Library/Group Containers/UBF8T346G9.Office/Outlook/Outlook 15 Profiles/

Before deleting or rebuilding anything, I would strongly recommend verifying whether the user has any On My Computer folders or other local-only content that is not stored on the server.

At the moment, I am not aware of a published Microsoft known issue that specifically matches:

Outlook for Mac 16.112.3
macOS Tahoe 26.6.2
A mailbox that repeatedly stalls or restarts synchronization around 12,000-15,000 messages

Because the mailbox works in webmail, mobile, and previously in New Outlook, I would be inclined to focus less on rolling back Outlook and more on determining whether the Legacy Outlook profile can be safely rebuilt.

If this is an Exchange Online mailbox and there is no important local-only data, creating a new Outlook profile would be one of the most practical troubleshooting steps to determine whether the issue is related to the Legacy Outlook synchronization database. While I would not consider a local profile issue to be the only possible explanation, it is a relatively low-risk test in an Exchange Online environment.

One final point worth considering: Microsoft has announced that Legacy Outlook for Mac will reach end of support for Exchange Online in October 2026, and New Outlook is now the primary client moving forward. If the mailbox continues to function normally in New Outlook, it may also be worth considering whether remaining on Legacy Outlook is necessary for this user.

I hope this information helps point you in the right direction.

Warm regards,

If the answer is helpful, please select "Yes, accept answer". If you have any questions or new updates, please feel free to comment below.

Vinud Aslama 0 Reputation points

2026-09-15T11:05:13.81+00:00

Hi, thanks for your answer and suggestions. We are using IMAP for this outlook. Though I am not too familiar on the local profiles/data in Mac on how to locate where this mail files are downloading. One of the technician who checked it was not able to locate as well as he recommends that maybe there are corrupted files upon downloading/synchronization so we have to delete it but since he was not able to resolve it, that's why I am checking any possible problem or solutions on this one.
Liora 1,720 Reputation points Independent Advisor

2026-09-15T13:58:41.82+00:00

Dear Vinud,

Thank you for the additional information.

Since this is an IMAP account, I would recommend keeping the existing profile intact and creating a new Outlook profile for testing. This is the safest way to determine whether the issue is related to the Legacy Outlook synchronization database without risking any existing local data.

If the mailbox syncs successfully in the new profile, that would strongly suggest the issue is related to the current Legacy Outlook profile rather than the mailbox itself.

I would also encourage testing the mailbox in New Outlook. If it works reliably there, it may be worth considering New Outlook as the long-term solution, as it is the actively developed client and Legacy Outlook for Mac is approaching end of support.

Warm regards.
Liora 1,720 Reputation points Independent Advisor

2026-09-16T10:53:47.08+00:00

Hi Vinud,

I hope you're having a great day.

I'm following up to see if you've made any progress with this issue or if there are any updates you'd like to share.

Wishing you all the best, and I look forward to hearing from you.

Kind regards.
Vinud Aslama 0 Reputation points

2026-09-16T10:58:31.19+00:00

Hello. Thanks for following up. In the same computer, there are 4 IMAP accounts logged in. But only 1 email is having synchronization errors. Maybe because it has a larger mailbox count compared to the others which easily syncs. But this is also happening in the other Macbook. Just for the information, all our IMAP accounts are working well, (in mobile and desktop). Only this specific account in the Mac is having this problem (please take note that this account is working well on the mobile app also).
Vinud Aslama 0 Reputation points

2026-09-16T11:14:56+00:00

Hello. Thanks for following up. In the same computer, there are 4 IMAP accounts logged in. But only 1 email is having synchronization errors. Maybe because it has a larger mailbox count compared to the others which easily syncs. But this is also happening in the other Macbook. Just for the information, all our IMAP accounts are working well, (in mobile and desktop). Only this specific account in the Mac is having this problem (please take note that this account is working well on the mobile app also).
Liora 1,720 Reputation points Independent Advisor

2026-09-17T11:30:47.1333333+00:00

Hi Vinud,

Thank you for the update.

Since the same mailbox exhibits similar synchronization behavior on multiple Macs while the other IMAP accounts continue to synchronize normally, the issue no longer appears to be limited to a single Outlook installation.

If you create a new Outlook profile and the mailbox still stalls or repeatedly restarts synchronization, I would recommend opening a Microsoft support case. At that point, the issue may require analysis of Outlook synchronization logs or mailbox-specific IMAP behavior that is not visible from the community side.

You can raise support ticket from Microsoft 365 Admin Center>Support>Help & Support.

For reference:  Get support | Microsoft Docs  

I hope you are able to identify the root cause soon. If you receive any findings from Microsoft Support, please consider sharing them here, as they may help others encountering similar behavior.

Kind regards.
Vinud Aslama 0 Reputation points

2026-09-17T11:40:35.3366667+00:00

Thanks for your update. Actually, it only happens to this one specific email account and on Mac. While it's working well on the mobile. We've tried switching of the legacy outlook and moving to the new outlook for us to see if it will resolve the issue. As of the moment, we are still waiting the sync to finish after we've went back to the previous outlook version being used before the sync problem happened.
Liora 1,720 Reputation points Independent Advisor

2026-09-17T15:28:09.6533333+00:00

Hi Vinud,

Thank you for the update.

Hopefully the mailbox will sync successfully and help narrow down whether the issue is related to Legacy Outlook for Mac or something specific to this mailbox.

Please keep posted on the results. I'd be happy to continue helping if you discover anything new or need assistance reviewing the next steps.

Kind regards.