Hi SASPro,
Thank you for using the Apple Support Communities! We understand Mail is using an unexpected amount of CPU on your Mac and causing some overheating issues. Let's work together to see what we can identify to resolve this.
-When did you first notice this issue? Was it after any changes to your Mac or email accounts?
-What type of email accounts do you have set up in Mail?
If you have multiple accounts set up in Mail, we'd like to have you disable (not remove) each email account, and then enable one mailbox at a time and monitor to see if the behavior relates to one specific mailbox, or all: Choose Mail > Preferences, click Accounts, select the active account, click Account Information, then uncheck “Enable this account.”
If the issue is not specific to one account, or you only have one email account set up in Mail, let's have you boot up into safe mode. It’s a way to start up your Mac so it performs certain checks, repairs, emptying of system cache, and prevents some software from automatically loading. This will let us know whether issues you’re experiencing are caused by software that loads as your Mac starts up: How to use safe mode on your Mac. Once in safe mode, check to see if the same behavior continues in Mail and Activity Monitor. Sometimes safe mode can successfully resolve some issues. With this being said, after testing in safe mode, restart and log in as normal to see if that helps resolve the issue.
Keep us posted. Regards.
