"Wrong or missing login data to access mail transport server smtp.mailbox.org."
After I slightly change some non-critical settings of my mailbox and then try to send any message, I get the following error:
"Wrong or missing login data to access mail transport server smtp.mailbox.org. Please check associated account's settings/credentials."
Noteworthy, that I don't touch any settings related to SMTP or forwarding, don't setup an external client (do everything in the web interface) and even if I return everything back, the error continues to appear.