MAIL in Monterey 12.5.1 has many problems

Hi Graham Giles, 

Thanks for connecting with us here. You've mentioned that you're having multiple issues with the Mail app on your Mac. The first thing we can have you try is rebuild your mailbox: Rebuild mailboxes in Mail on Mac - Apple Support

Rebuild mailboxes in Mail on Mac

You may need to rebuild a mailbox to update the list of messages it contains. For example, if messages appear to be missing or garbled, or if you can’t find all relevant messages when you search using the Entire Message search option.

1. In the Mail app on your Mac, select a mailbox in the sidebar.

2. Choose Mailbox > Rebuild.

Let's also check that your Mail settings are correct for your email accounts. You can use the Mail Settings Lookup tool to check this: Mail - Official Apple Support If you can't find the settings for your email service provider using that too, you'll need to contact them directly. 

Starting up your Mac in Safe Mode is also a good step to try since it can help determine if the issue is caused the software that loads when you start up your Mac. See: How to use safe mode on your Mac - Apple Support

"How to use safe mode

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

1. Shut down your Mac.
2. Turn on your Mac and continue to press and hold the power button until you see the startup options window.
3. Select your startup disk, then press and hold the Shift key while clicking “Continue in Safe Mode.”
4. Log in to your Mac. You might be asked to log in again.

Intel processor

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
2. Release the key when you see the login window, then log in to your Mac. 
3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window."

Have a great day.

Good afternoon Graham,

It might be helpful for you to speak with an AppleCare technical advisor about this issue. Click the link below, choose the phone number for your region from the list, and call when it is convenient.

Contact Apple for support and service - Apple Support

Take care.