Apple Mail delays with Microsoft Exchange email
I've been experiencing significant delays in performance with my Microsoft Exchange email, when using Apple Mail as a client. I'm running macOS 15.3.1 on an Apple Silicon Mac. The account is set-up via Exchange. This is an email mailbox that was migrated from another IMAP service provider to M365, last year. Ever since the migration, performance has been dismal.
- Mail Activity is perpetually slow in downloading messages (as per the activity window, currently 700 of 150,000)
- I consistently receive an error message, when applying rules for moving messages into various mailbox folders. "The message could not be moved to the mailbox "XXXX". Invalid base64 string for MIME content."
- I've tried rebuilding the mailbox index several times. I've deleted the index via Onyx Mac.
- The Microsoft Outlook app seems to display the email, without issue; however, I'm hesitant to move to this default app, without understanding why Apple's apps are not functioning at a high level of performance??
Any help or advice would be appreciated. I'm a seasoned Mac user, with advanced knowledge, so am not afraid to try.
Mac Studio, macOS 15.4
