What conditions trigger IMAP synchronization in New Outlook?

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What conditions trigger IMAP synchronization in New Outlook?

Arnor Chen 20

What conditions trigger IMAP synchronization in New Outlook? It continuously sends SELECT commands to the IMAP server (SELECT INBOX) and the server responds with increments in *EXISTS and *RECENT, but it just does not FETCH new messages. Why?

ps: I am mail server author.

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Answer 1

Hi @Arnor Chen,

Thank you for posting your question in the Microsoft Q&A forum.

Based on my research, Microsoft intentionally shifted IMAP in New Outlook to a hybrid, demand-driven model rather than a traditional, event-driven IMAP client:

IMAP in New Outlook is used primarily for folder state and metadata (EXISTS/RECENT as “heartbeats”), while the actual message bodies and full sync are off-loaded to Microsoft’s cloud infrastructure.
A FETCH command is only sent when the client truly needs the message content. When you open or scroll into a folder, when metadata shows the message isn’t cached, or when you explicitly invoke Sync now.
If your account is using Microsoft’s sync layer (the default for non-Exchange/Graph-native accounts), the IMAP commands you see are largely keep-alive checks, not fetch triggers.
New Outlook does not fire off FETCH on every EXISTS/RECENT change, nor does it continuously UIDNEXT/UIDVALIDITY check like a full-fat IMAP client. Without support for server extensions like IDLE, CONDSTORE or QRESYNC, Outlook defers downloads until the next interactive or scheduled sync window.

Essentially, this behavior is by design: IMAP is no longer the primary sync engine in New Outlook, and message retrieval is demand-driven, not event-driven. Additionally, New Outlook does not offer user-configurable options to adjust IMAP sync intervals. All sync scheduling is managed internally by Microsoft and cannot be customized from the client side.

Please note that as a forum moderator, I do not have access to internal development tools or escalation channels. However, I hope the information above helps clarify the behavior you're seeing.

Thank you for your valuable time. If you have any further questions or observations, feel free to leave a comment under this post. I’ll do my best to assist within my capacity.

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Answer 2

Arnor Chen 20

In my research, I already found some mechanisms of new outlook.

I got my IMAP server to disable IDLE command, new outlook will work well when you click SYNC.

Therefore, if something went wrong with IDLE command, it will affect next behavior including new mail to list.

And I suggest to not sort message, the "Sort messages to Focused and Other" is annoying and stupid. How can it determine which letter is the focus or the other? I just can't find latest mail because it went to other panel.

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Answer 3

How to make "SYNC" to work again without delete/re-create account?

Here is my experience.

If you are facing new outlook won't sync issue, you could try follows:

wait for next day. I don't know why but time could heal new outlook.
go to account management to press "FIX" and wait for miracles happened.

If there is no miracle, just delete and re-create it again.

Although it may start to receive new mails, but those non-existing (already deleted by other mail clients) mails may still remain on the list. That's why a correction function "FORCE SYNC" or "COMPACT" is needed. Or MS cloud could add something like that to self fix.

Answer 4

After few hours testing, the unable sync issue occurs again.

My test scenario is multiple devices to the same IMAP account.

I use Windows Live Mail(as A), ThunderBird(as B), New Outlook(as C) to connect the same IMAP account of my server. (this account has thousands mails in INBOX and sub folders.)

Add/Delete mail in A, observe the others.

Add/Delete mail in B, observe the others.

Add/Delete mail in C, observe the others.

Suddenly New outlook becomes unable sync, even press SYNC. The others are working well. I don't notice what action cause that. (maybe delete in A, then found C didn't change, then C won't change anymore. Thus, the issue may be occurred by IDLE command).

I think maybe New outlook needs a "FORCE SYNC" to fully check by UID due to it may not prepare for multiple devices. When something went wrong as it dones't expect, it will only do useless SELECT cmd recursively.

My server log

2025/09/27:12:20:39 [1324]40.99.93.141 <<<< WIR00003 OK LOGIN Completed

.

2025/09/27:12:29:28 [1324]40.99.93.141 >>>> WIR06072 SELECT "INBOX"

2025/09/27:12:29:28 [1324]40.99.93.141 <<<< WIR06072 OK [READ-WRITE] SELECT Completed

From 12:20 to 12:29 , it sent SELECT command for 6xxx times but my New outlook mail list is still the same. The list is freezed. Press SYNC won't change anything.

Rin-L 19,335 Reputation points Microsoft External Staff Moderator

2025-09-27T06:10:18.9+00:00

Hi @Arnor Chen,

New Outlook primarily uses IMAP to maintain folder state. In cases where the client state becomes desynchronized. For example, due to rapid changes across multiple clients or an oversized inbox, it may get stuck in a loop of SELECT commands without issuing UID SEARCH or UID FETCH. In such scenarios, the Sync button may not help, which aligns with the “demand-fetch” design of New Outlook.

I understand that product design may not always fully accommodate every user scenario. As a forum moderator, I hope you understand that I don’t have the ability to directly modify or escalate product issues. However, users like you can absolutely share your ideas and suggestions, like requesting a FORCE SYNC feature to fully check by UID via the official feedback portal: Outlook (new) · Community

This is the official channel where your feedback goes directly to the product team and can be considered for future improvements.

Thank you again for your understanding and collaboration.

Answer 5

Arnor Chen 20

I finally delete the account (from all devices) in new outlook and re-create the same account.

It works well as it should be, but I wonder when it will not sync at all again.

Becuase before I got this issue, it works well, too.

I will keep observing the sync issue.

Rin-L 19,335 Reputation points Microsoft External Staff Moderator

2025-09-27T00:30:26.69+00:00
Hi @Arnor Chen,

Thanks for sharing your update.

Deleting and re-adding the account helps because it clears the local cache and resets any corrupted sync state in Microsoft’s sync layer. When you reconfigure the account, Outlook rebuilds everything from scratch, so FETCH commands start working again as expected.

The issue likely occurred because New Outlook depends on both local cache and cloud sync logic. If that sync state gets misaligned (due to network interruptions, stale UID mappings, or internal bugs) Outlook may stop fetching messages even though it still sees EXISTS/RECENT updates. This behavior has been seen in some builds.

Unfortunately, it can happen again, especially if:

The account uses IMAP without extensions like IDLE, CONDSTORE, or QRESYNC.

There are long-running sessions or unstable connectivity.

The sync layer encounters a mismatch it can’t self-correct.

To clarify, this is not caused by your IMAP server, but by how New Outlook manages sync internally. Re-adding the account simply resets that internal state.

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What conditions trigger IMAP synchronization in New Outlook?

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