If you add a To Do item when offline, Mail.app might crash repeatedly when you go online, with no way out of the crashing loop (!).

I found the solution to this nasty Mail.app crash here.

Start Terminal application: it’s in Applications/Utitlities/ folder. In the following, do not type the quotes, just type what’s in the quotes.

Go to ~/Library/Mail/IMAP-email_address@mailserver/.OfflineCache like this

"cd ~/Library/Mail/IMAP-email_address@mailserver/.OfflineCache“

and list the contents like this

"ls -al“

Then delete all the files with numbers, like this

"rm 1“

WARNING: this worked for me. If you’re not comfortable with any of this, don’t do it. Back up the .OfflineCache folder first if you’re uncomfortable, but brave.