Q: MAC Mail no longer connects to my mail server after 11/30 yosemitee update.

I've had the same problem. I use Yahoo and it states on the Yahoo site it has never supported other mail programs. Any thoughts on separate apps to combine multiple emails?

Mail troubleshooting - Yosemite

What does Mail/Window/Connection Doctor Show? If the server is red, select it and look at the Show Details box.

Troubleshooting sending and receiving email messages

Troubleshooting sending email messages

SMTP servers keep going offline

After updating to yosemite  and watching my imac slowly become less and less functional it finally imploded completely.... resulting in a new hard drive (maybe a coincidence but an odd one) After a complete reinstall of yosemite and all my data nearly everything works except for mail.  Many people have suggested unclicking a few boxes in mail preferences  but what do you do if you can't even open mail preferences?   Deleting and re-adding the account has not worked for me either.