Sending failed; Reason:503 5.7.0 encryption too weak 0 less than 39

Sending failed; Reason:503 5.7.0 encryption too weak 0 less than 39

I connect my outgoing mail to another mail server which requires authentication. As far as I know there have not been changes to that mail server. But now, as of this morning, I get the following when trying to relay mail through that sever. It has been working fine until today.