Sent Mail frequently not up to date in Mail app on macOS

OS: macOS Catalina 10.15.7

Application: Apple Mail 13.4 using IMAP/SMTP

I have an issue with Zoho Mail and the macOS Mail app. Below is a bulleted list summarising the problem:

- My sent mailbox in the Mail app often has 67 emails missing

- The 67 missing emails seems to be the latest 67 sent emails

- I have to hit the “Get Mail” button to reload these missing sent emails

It can take quite a few minutes each time to reload all of these emails. All attachments need to be re-dowloaded too.

The missing sent mail only seems to be missing in the Mail app, and is always present when viewing my Zoho Mail sent mailbox in a browser.

Sometimes after reloading these 67 missing emails they will go missing again when I return to view the Sent mailbox. This can happen a few minutes later, or after a few hours, and happens multiple times a day.

I had not updated macOS or the Mail app when this problem started. Since then I have only installed a security update on macOS, which made no difference (as I understand it, Apple often bundles smaller OS updates within their security updates)

The problem started earlier this year, around the same time as I started receiving the error detailed on the following post. Even though the error mentioned in this post has stopped, the issue of missing sent emails continues, so I’m unsure if they’re linked.

https://help.zoho.com/portal/en/community/topic/what-does-this-mean-and-how-to-solve-it-the-imap-command-%E2%80%9Cappend%E2%80%9D-to-sent-failed-with-server-error-append-failed

This problem can be frustrating when reading through email threads with my clients, as I cannot easily reference what they’re responding to as my emails are regularly missing from the thread. It can also be a bit disconcerting as I sometimes second guess whether I’ve sent an email or not, as they can sometimes be missing from the Mail app soon after being sent.

Coincidentally (or not) my wife also uses Zoho Mail and the macOS Mail app, and she has the same problem which started at a similar time, except the number of missing emails for her is always 90. She is using a different macOS version, and a different Mail version, and also hasn’t updated since before the problem started.

Because of the problem existing for both myself and my wife, on different app and OS versions, with no updates or changes to settings being made before the problem started, I can only deduct that the problem is on Zoho's side. If anyone has any info, advice or a potential fix, I’d very much appreciate it.