I have 2 different email servers for 1 domain.
I set up 2 MX records with different priority, in order to always receive emails when 1 of the servers has problem.
The first server using another email system, and the second server uses MailCow
When I log in to the user in first server, and try to send emails to the users in second server, I get this message:
Sender address rejected: not logged in
How to solve this problem?