Problems with Mail after updating Tahoe 26.6.2

That rules out the Inbox size. With only 30–40 messages and the IMAP limit already set to 1,000, I wouldn’t keep rebuilding Mail.

The other important clue is that your second Gmail account works, while this particular Gmail account is slow on the Mac, iPhone, and iPad. That points more to an account-specific Gmail/Apple Mail synchronization problem than to a damaged Mail database on the Mac.

One low-risk check I’d make in Gmail webmail is Settings > See all settings > Labels. If that account has Gmail labels you don’t need in Apple Mail, turn off Show in IMAP for those nonessential labels and let Mail resync. Don’t change Inbox, Sent, or Trash. Google specifically recommends reducing the folders exposed to IMAP when a mail client is syncing slowly:

https://support.google.com/mail/answer/7126229

Also, on the Mac open Window > Activity, then try deleting or moving one message from that Gmail Inbox. If an operation sits there for a long time, the exact wording shown in Activity would be useful to know.

Since the same problem is happening on your Mac, iPhone, and iPad, I wouldn’t assume Tahoe 26.6.2 is the cause yet. That points more toward the mail account or provider.

I’d check the account directly in your provider’s webmail first. See whether new messages appear normally and whether you can delete a message there. If webmail has the same problem, the issue is with the account/provider rather than Apple Mail.

If webmail works normally, which provider is this — iCloud Mail, Gmail, Outlook/Exchange, or something else? That would help narrow it down.

I also wouldn’t keep removing the account or deleting Mail database files until you know all mail is safely stored on the server.

Apple recommends the same webmail check here: If you can’t send or receive email on Mac - Apple Support

Thanks, that helps narrow it down.

One important distinction: if those mailboxes appear under iCloud in Mail’s sidebar, they aren’t part of the Gmail account. Messages moved there are being stored on Apple’s iCloud Mail servers, not Google’s.

Since Gmail works normally at gmail.com, I’d next sign in to iCloud.com/mail and check one of those affected mailboxes there. See whether the messages are present and whether you can move or delete one normally.

If Gmail webmail and iCloud Mail both work correctly, but Apple Mail has the same synchronization problem on the Mac, iPhone, and iPad, I wouldn’t remove the account or delete Envelope files again. At that point it would be worth reporting to Apple Support as a Mail synchronization issue.

One question: in Mail’s sidebar, are the problem mailboxes actually listed under iCloud, or under the Gmail account? That distinction matters here.

Thanks, that makes the picture much clearer.

If the iCloud mailboxes and the mailboxes “On My Mac” work normally, while receiving, moving and deleting is slow specifically in the affected Gmail Inbox, I would stop troubleshooting the local Mail database for now.

One thing I’d check next is the size of that Gmail Inbox. Roughly how many messages are in it?

Google specifically notes that very large IMAP folders can make third-party mail clients slow and provides an option to limit how many messages per IMAP folder are synchronized. If your Inbox contains a very large number of messages, that would be a useful and reversible test before removing the account or deleting Mail files again.

So for the moment I wouldn’t change anything else — the approximate Inbox size would be the most useful next piece of information.

It all points, as you say, to a problem with the Gmail account. The fact it works OK via webmail is sort of irrelevant because the way webmail interacts is different from the way a mail client works. It's also a Google product.

You could try another mail client to help isolate the problem. Thunderbird is free. If the problem Gmail account works fine with Thunderbird then it narrows it down to an Apple Mail/Gmail issue. If Thunderbird has the same problem then it's the Gmail side.

Thanks for the screenshot. That’s Activity Monitor, which shows how much CPU and other system resources Mail is using. It won’t show us what Mail is doing with the Gmail account.

With Mail open, choose Window > Activity from the menu bar. Then try moving or deleting one message from the affected Gmail Inbox.

If an operation hangs there, a screenshot of that Mail Activity window — or the exact text it shows — would be useful. No need to force-quit anything.

Understood - that does indeed narrow it down to an Apple Mail/Gmail interaction.

Many years ago I had a similar problem - Mac Mail simply wouldn't remember the password for one of my mail accounts. Not Gmail, but I had other accounts on the same server which worked fine. In the end I solved it by deleting all entries in Keychain for all the accounts on that server and set them up again but I see you've tried that.

When you set up the account for the first time did you make sure that the SMTP side worked before you set up the IMAP side? I've had that problem before with one of my accounts.

Have you confirmed that the SSL settings are identical on the working and non-working gmail accounts? Ditto the IMAP settings? I'm sure you've tried everything but just in case.

Best of luck. This would drive me mad.

Have been working with Apple Support since 31st July to resolve mail issues without success. This is an iCloud mail account. On today's call they advised me that Apple is unable to provide a solution at this time.