I have recently changed my server from Windows based Plesk mamnaged system to Linux based Plesk management system, since doing do a lot of my customers who use pop are getting bounce back emails from various clients they send to.
See below the error message.
This Message was undeliverable due to the following reason:
Your message was not delivered because the destination computer refused to accept it (the error message is reproduced below). This type of error is usually due to a mis-configured account or mail delivery system on the destination computer; however, it could be caused by your message since some mail systems refuse messages with invalid header information, or if they are too large.
Your message was rejected by mail.rotra.com for the following reason:
5.7.1 Sender ID (PRA) Not Permitted
The following recipients did not receive this message:
I think its something to do with the spf record in the dns settings for the domain.