Hello,
1. Setup O365 media type with use of MS365 tenant which does have default settings,
2. try to test media type

in a result it would fail, if MS365 tenant does use default settings
error shown in frontend is:

Login denied (possible manual token revocation). Please reauthorize from frontend.

this is example message which would be logged in zabbix_server.log (alerters processes should have at least debug level 4):

0000:00000000:000000.000 < 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Tenant. Visit https://aka.ms/smtp_auth_disabled for more information. [xyz.POLP291.PROD.OUTLOOK.COM 2025-07-03T00:00:00.416Z xyz]

Expected:
To solve this it's needed to enable SMTP client auth either per user/shared mailbox or per whole tenant in MS365,
I was not able to find any note about this in Zabbix documentation, thus it would be great to see it, since error from frontend is bit misleading to what is an issue,
https://www.zabbix.com/documentation/current/en/manual/config/notifications/media/email/gmail_office
https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/authenticated-client-smtp-submission

thank you,