Thunderbird email - outgoing server problem

I know there's some sort of "Comcast Email Migration".  But I only use IMAP with Thunderbird and I don't use Outlook.  According to the e-mails Comcast has sent me over the last two months, if I don't use Outlook, I don't have to do anything:

"If you do not use Outlook to read your emails, please disregard this notice." I did ignore the messages and That is NOT true!

As of Tuesday morning 12/19/2017, Comcast disabled the outgoing SMTP server that I had been using - smtp.ch3.comcast.net. Now I can receive emails, but I can NOT send out email. I spent an hour on the phone this morning, and got them to set up my Office365 Business Essentials account. I now have the username and password set up for the outgoing mail server, but it doesn't work. I get the message "Login to smtp.office365.com failed". Then the dialog box says "Please verify that your Outgoing server (SMTP) settings are correct and try again".

I have spent a long time doing various Google searches to figure out why it is failing, but so far no success.