When trying to send an email message from a server to another server I receive the following message:

SMTP error from remote mailer after MAIL FROM: SIZE=2412: host mydomain.com [209.51.***.***]: 451 rejected: temporarily unable to verify sender address (try again later).

Could you tell me what's the cause of this error ?

Thank you.

This could be if your sending domain's DNS is down.

The remote SMTP is trying to verify the sender's DNS and if it returns a time-out or lameserver this message can occur