I have a VPS in which a client website is hosted. We never had trouble with email, but recently we received an message from someone who tried to register at the website and didn't receive the confirmation email (sent via PHP's mail()). He claims to have made it to work by adding our domain to his whitelist.
He runs his own server, and also mentioned that he was watching his logs and the email came from dreamhostps.com and was automatically rejected since this domain has no MX record. The person also claims that this is a trick spammers use but easily fixed by legitimate senders.
I'm afraid I did something wrong. The server was auto configured by DH panel, and we use Google Apps so I'm sure the MX is set to Google's domains. Shouldn't anyway the MX be used for incoming mail? I'd expect the outgoing mail to come from dreamhostps.com in fact.
I have little experience with MX and related subjects. Thanks in advance if someone can give me a clue of what could be wrong in this case or maybe if the problem is on the other side (the guy running his own server).