Got several mailboxes made in one batch. Identical, default configuration.
One of the users cannot send anything via SMTP - he gets
5.7.60 SMTP; Client does not have permissions to send as this sender.
This user has no problem sending same mails from OWA.
Also, adding an explicit
Send As permission solves the problem with SMTP, but it's not required with other users - they send via SMTP without any problems.
What can be wrong?