Videotron email not working in Microsoft Office 365 Outlook for Mac

Hi JW787 From the SMTP error code 550 5.1.0, this indicates that the sending is completely interrupted instead of being retried. Then the code EML-055 indicates that the antispam filter is blocking the sending because the email used for sending is not a Videotron address. You would need to access your Outlook 365 settings and make sure the email address configured does not have any errors.  It would also be a good thing to check your SMTP server settings and to make sure the option "My outgoing (SMTP) server requires authentication" is enabled". The secondary option to use the same information as the incoming mail server to authenticate that is enabled by default can be left as it is.