ERROR WITH SMTP!! (HELP)

Hello,
A client of mine uses a standar pop3/smtp account outside of my network. Using my network, when he sends an email he gets this message:

Relaying denied. IP name possibly forged [MY PUBLIC IP HERE]

This problem started a few days ago

Anyone haves a clue about this issue?

Thanks!

please check if you are not transferring all outgoing SMTP requests to your SMTP server. If not, ask your client to check if his SMTP server supports authentication, and if so, if he is authenticated before sending the e-mail. The rest should be resolved with his/her e-mail service provider.