"550 Relaying denied. IP name possibly forged" error when sending mail

> I am getting these messages from time-to-time. Any idea why? These are
> addresses that I have sent emails to before, without a problem. 

> From: Mailer_Daemon@example.com
> Sent: 13 July 2016 18:10
> To: Postmaster@example.com
> Subject: Delivery Status (Message Send Error)

> Your message (ID:) could not be delivered to the recipients below See the
> delivery status below for more information
> - joe@example.net

> VPOP3 was trying to send a message to the server "smtp.example.net" when it
> encountered errors.
> These errors probably originated from that server

> Whilst trying to send to "joe@example.net", VPOP3 received the error "550
> (5.7.1 ... Relaying denied. IP name possibly forged [])"

The "550 Relaying denied. IP name possibly forged" error is coming from the remote server, not from VPOP3. A quick search on Google for that error text suggests that this error may be if you don't have DNS set up correctly.

Because you have VPOP3 sending using 'SMTP Direct' rather than going through an ISP's relay server you need to ensure that you have DNS set up correctly for your server. This option is much more difficult to get right than going through an ISP's server.

At the least, the VPOP3 host name (set in Settings -> Misc Settings) should match the DNS name of your VPOP3 server, and there should be a reverse DNS entry for your IP address which matches that name. (You should have a static IP address as well). If you can't do these things, then you should probably change VPOP3 to send mail through an ISP's SMTP relay service, otherwise you will have problems sending mail to some domains.