I have previously setup a Gmail alias that allows me to "send as" my office365 email address. This is achieved in Gmail under "Settings", "Accounts and Imports" and "Send Mail As'". I have successfully set this up with multiple .edu outlook accounts, including my current one, and it worked well until 2 weeks ago. My other .edu outlook accounts continue to work. Only one does not work.
When I enter in the SMTP settings:
Sever name: smtp.office365.com
Encryption method: STARTTLS (Gmail only has options for TLS or SSL- I don't know if this is the source of the problem)
I receive the following error message: Authentication failed. Please check your username/password.
Server returned error: "334 VXNlcm5hbWU6 334 UGFzc3dvcmQ6 535 5.7.3 Authentication unsuccessful [MN2PR13CA0006.namprd13.prod.outlook.com] , code: 535"
However, my username and PW are correct. Some have suggested pinging smtp.office365.com to find the specific server I am using, but this hasn't worked. Others (https://support.google.com/mail/thread/38336515?hl=en&msgid=39890656) recommend contacting you or the administrator because Google is insisting on the use of genuine, correct certificates for servers. I have also seen that it might be related to two-factor authentication.
Please let me know if there's any additional information you might need to help resolve this.