Open Mail, go to Preferences
Under the General Tab, set the default mail reader to Thunderbird.
Under the Accounts Tab, make sure there are no accounts that are active.
Should fix your problem
Hi everyone, sorry if this is an old topic, but I really don't even know what to call it in order to search for it.
My problem is this, I use Google Calendar, and therefore chose to sync my OS X Calendar with my Google Calendar, all in all I quite like that it will send me little popup notifications on my desktop and that I can see/edit my calendar using the Mac program. For me the downside is that I don't use the Mac OS X Mail client, but rather use Thunderbird. It seems though, that Mac Mail recognizes that I've synced Calendar to my Google account and wants to be my email client and send me additional reminders of things that pop up on my calendar. Can someone help me convince Mail to pipe down, and preferably never bother me again?