E-Mail Not sending

I upgraded to Yosimite and now I can't send. Message is username or password is wrong...but it's not. Using Telus isp and Apple mail. IT'S DRIVING ME NUTS!

This is a typical Apple screw-up - they introduce a new option : 'Automatically detect and maintain account settings' and by default have it turned on - thereby breaking stuff that previously worked. Turn this option off, and your mail accounts should then work as they did before upgrading to Yosemite. For incoming mail, this option can be found in the Advanced Tab in Mailreferences:Accounts. There is also a corrisponding setting for outgoing mail in Mailreferences:Accounts:Outgoing Server:Edit SMTP Server list:Advanced.

My solution, don't use Apple Mail! I think even Thunderbird is better and AirMail is so much better Apple Mail pales in comparison. It's sad to think I should have to suggest alternative email clients, I have tried each version of Mail on each new OS incarnation and continue to be disappointed with it but if luvinlex's suggestion works and you are satisfied with Mail in every other way by all means take that path. Thanks luvinlex.

mac mail

I've never used anything but mac mail... I wouldn't know where to begin learning a new one. It works fine for me. I have not done any upgrades after Lion came out. It blocked all my old 3rd party software, like basics. Adobe Illustrator, Office for Mac, etc.. Each said I needed to upgrade by buying new software at it's retail price. I fought hard with Apple and got a designated technician. We worked on the phone for a week getting it partitioned and rebuilt my old system back from my back up. ( which I plan to do now )

I'm glad to hear it. I have to say I have had nothing but positive experiences with Apple Support. I and good on you for having a Backup in the first place.