Outgoing mail (SMTP) brokem: Sender address rejected: not owned by user ...

Since this morning (or somewhere last hours) all my outgoing mail of several addresses are broken. I see the following error when sending out an email on one of those accounts:

Sender address rejected: not owned by user {opalstack_mailbox_name}. The username (mailbox name) is correct, and it also linked to the email address used in the control-panel.

I did not change anything in the configuration, all the accounts used to work fine until now. This happens on 3 accounts at least.

Did anything change and do I need to update the mail settings? Receiving email still works for these same accounts.