i have now generally searched for current oauth problems with other tools.
It is repeatedly pointed out that smtp is deactivated by default in M365 and must be activated manually, here for example:
"Change in authentication
Microsoft made the following changes:
The option for standard authentication using a user name and password has been removed; OAuth authentication is now required instead.
In some cases, SMTP authentication has been completely disabled. SMTP authentication is always disabled by default for new accounts. There are also some restrictions for SMTP that are not yet known."
Activate SMTP authentication. You must do this in two places:
for the entire organization: in the Exchange Admin Center under Settings > Message flow > Security > Check mark
for individual users: in the Microsoft 365 Admin Center under Users > Active users > click on a user > Email > check Authenticated SMTP
