"that domain isn't in my list of allowed rcpthosts" error

[sempiterna]

I have received a few complaints from people who have been receiving the following error from my zimbra server when sending mail to my account on the Zimbra server:

xxx.xxx.xxx.xxx does not like recipient.
Remote host said: 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
Giving up on xxx.xxx.xxx.xxx.

I have been receiving a lot of mail just fine since i installed zimbra, but the fact that some people seem to receive this error makes me think that the system is actively blocking mail from some senders.

I did not add any domain to any blacklist. What can this be?

[XenoPhage]

Are they sending mail to an alias domain, perhaps, that isn't set up in zimbra? For instance, if you have example.com and mail.example.com set up in DNS, but only example.com is configured in zimbra, it may reject anything sent to user@mail.example.com ...

[sempiterna]

No, no alias domain. I do receive a lot of mail, but less than i used to when i was still using qmail. I only get those error reports from people who take the time to mention it to me, so i have no idea how large the problem is.

But the question really is, why is a lot of mail arriving perfectly fine, while some are rejected by my zimbra server with this specific error? Also, is there a way for me to capture all of these errors?

[XenoPhage]

You should be able to find these errors in /var/log/maillog