Mail issue on my MacBook Pro after Sequoia 15.3 update

I was having issues on Sequoia 15.1 with my boss's email in Mail updating especially calendar events for his Microsoft Exchange email. It was suggested as a first step to upgrade to the latest OS and now his emails are disappearing while he is working on them, and then they will come back again.

I don't know how to assist him. I had him turn off the new AI intelligence feature, but it is still not working correctly.

Any ideas?

[Re-Titled by Moderator]