After switching to Microsoft 365, you need to adjust the login details in the email account. The sender address must match the logged-in account exactly, otherwise Microsoft discards the email without an error message.

How to proceed

1. Open Setup » Client » Global » Email accounts.
2. Enter smtp.office365.com as the SMTP server, port 587 with TLS (or as specified by Microsoft).
3. Check whether the sender address of the email template matches the SMTP user exactly.
4. Check whether the account is assigned as the default sender account.

More information: How do I enter the email server access data in PlentyONE for sending emails?