Exchange SMTP Error

We run Exchange 2003 SP2 on Windows 2003 SP1.
The Exchange box is behind and ISA 2004 with a server publishing rule.

Occasionally people who send us mail receive the following error…

: host mx.domain.com[mx ip] said: 550 Transfer blocked – error in processing (in reply to end of DATA command)

They could resend the same email again a few seconds later and it will go through. Also, in the most recent instance, it went to recipient1 but not recipient2.

Any ideas?