Can't send email to myself

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

From what it sounds like, it is unlikely Monterey is the cause of the problems

When you say "iMail" what exactly do you mean by this. Do you use the mail app, if so what account are you trying to send/receive it with? Is it an icloud account?

If so try to use icloud.com to send/receive email from there, I do recommend the same steps for any third-party email service as well what does happen from the web mail version. After isolating it from there and you do use icloud, it fails on the webmail, it is likely a storage-related problem with the icloud account