https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=38338 --- Comment #78 from David Cook <dcook@prosentient.com.au> --- After letting some time pass (and Brian adding an email subscription on the Microsoft account), I got a new error: 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox. Visit https://aka.ms/smtp_auth_disabled for more information. Now I can go into the "Microsoft 365 admin center", click on my user, click on "Mail", and then click on "Manage email apps", check "Authenticated SMTP", and "Save changes". That's fixed that error... and taken me back to the plain error: 535 5.7.3 Authentication unsuccessful -- You are receiving this mail because: You are the assignee for the bug. You are watching all bug changes.