some older Mail marked as unread

[jdw13]

I am using M1 Mac Air running latest MacOS Tahoe 26.0.1 and Mail Version 16.0 (3864.100.1.1.5).
Yesterday, I noticed many older emails from late August and early September* have been newly marked as unread.
The emails are on folders on the Apple Mail server, so perhaps something happened on the servers.
Has anyone else noticed this?

* Seems approx 24-Aug-2025 to 6-Sep-2025
Some of them could possible by deleted emails that reappeared.

 

[Bigwaff]

Yesterday, I noticed many older emails from late August and early September* have been newly marked as unread.

Select the mailbox in the left sidebar and then select menu Mailbox > Rebuild (at the bottom)

 

[jdw13]

Select the mailbox in the left sidebar and then select menu Mailbox > Rebuild (at the bottom)

When I do that, they are still marked as unread. Seems to be perhaps on the server, as I guessed.

 

[Bigwaff]

When I do that, they are still marked as unread. Seems to be perhaps on the server, as I guessed.

Did you log into the iCloud web portal and check if this is the case?

 

[jdw13]

Had not thought of that. Online I see only the current email, but the online header agrees with the local header reporting 14 messages and 14 unread (12 from late August- early September). Locally I see 14 messages, but online only 2.
Here is local on Mac and online:

 

[jdw13]

The issue persists after

1. Time Machine backup
2. use of Onyx to delete the existing Mailbox index in Mail
3. reboot
4. start Mail and wait for all the Mail to sync and get get reloaded

but if I mark exiting mail that I can see online read/unread on my Mac the status is quickly reflected in the Web view of my Mail.

I don't know how to see if the messages in question are on the Server but not visible or if they are only on my local Macintosh. They are at least local because I can view them on the Mail app locally on the Macintosh. If I delete one of the 'unseen' messages on my computer, the counter in the online web Mail view changes consistently with the Mac-locl view of the Mail.

Also, for messages viewable both on Web page and local Mail, I can mark read/unread from either end and both ends update and agree. Also if I delete such 'both visible' messages from either end, both ends update and agree but the 'web-invisible' messages deleted from the Mac just change the Mailbox counters.

P.S. Mailbox/Syncronize/iCloud does not change the status. Mail messages seen locally are still not seen on the web page mail https://www.icloud.com/mail/.