Mailing via SMTP in Joomla gives error "Could not authenticate" message for Office365
If you want to mail in Joomla via SMTP and you are using Microsoft 365 (Office 365) you may get a message "Could not authenticate".
As long as you've checked all the settings for the mail in Joomla via SMTP, you might want to take a look at the settings in Microsoft 365 because since April 2020, some important settings have changed for new accounts in Office365.
In your Joomla log file (you can find these files in: administrator/logs) you may see notifications similar to this:
mail Error in Mail API: SMTP INBOUND: "535 5.7.139 Authentication unsuccessful, the request did not meet the criteria to be authenticated successfully. Contact your administrator.
If you see nothing then you may still need to adjust your logging, these settings can be found in the General configuration on the logging tab.
Exchange Online / Microsoft 365 / Office365
If you are an administrator or want to get something modified in Office365, this article may be helpful : https://techcommunity.microsoft.com/t5/exchange-team-blog/securing-authenticated-smtp-in-exchange-online/ba-p/1293154
So the bottom line is that a default setting has been changed causing you to get the above error message.
For fans, a nice discussion in the forum : https://answers.microsoft.com/en-us/msoffice/forum/all/office-365-smtp-authentication-failing-even-with/4e2dcfc4-2626-4c3b-a945-9ff1b3404539?page=2
Another tip is to disable 2FA for SMTP if you use Office 365 to send your mail.