I found an old email from Yandex when I was texting to solve this problem. Here it may be useful.
The “Sender address rejected: not owned by user” error that occurs when sending a letter indicates an attempt to send a letter using someone else’s or another mailbox. There are two situations in which such an error can occur:
- In case the letter is sent with the indication of another mailbox, but within the same domain.
- If the letter is sent with the indication of a different mailbox using a different domain.
Please note: the error will indicate the mailbox in which the authorization was performed, and the mailbox that will be indicated as the sender:
(5.7.1 <email@example.com>: Sender address rejected: not owned by user firstname.lastname@example.org)
email@example.com - the mailbox in which the authorization was performed.
firstname.lastname@example.org is the mailbox specified as the sender.
My solution was that one e-mail was specified everywhere. This e-mail is registered on Yandex. Connect.