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?