Had a user forward me the undeliverable she recevied back trying to
send an email to an address she's sent to before.

The message that you sent was undeliverable to the following:

email@address.com (503 5.5.1 MAIL command expected)

Information about your message:
Subject: subject........not important
GroupWise Message Id: 517A6D49.B2F:123:45810
Message log tag: 479565
Number of send attempts: 1
Time of initial send attempt: 04-26-13 12:05:01

What gives? Something I need to check on my GWIA or elsewhere?

Running gw2012 sp1, gwia running on sles11sp1/oes11 (64bit) box.