550 #5.1.0 Address rejected

550 #5.1.0 Address rejected

I can send email from my ZoHo accounts to other external accounts just fine, so the SMTP server seems to be working. But any time I try to send an email from an external address to any of my ZoHo addresses I get the following error:

550 #5.1.0 Address rejected
Do I have something setup wrong, or is the IMAP server down for some reason?

-Eric Zollman