555 5.2.3 MailPolicy Violation Error

555 5.2.3 MailPolicy Violation Error

Hi, there. A friend of mine tried to send me an email but it got rejected and he received the above message as an automatic reply.
Would you please check what might have happened, since I'm terrified of having very important mails getting rejected by your server without me knowing about it?
Thanks in advance for your certain attention.
Bruno