[Mailman-Users] Unknown user- what am I missing?

[Zain Memon]

I have a nice list set up with Mailman and Postfix as my MTA.
Everything seems to work beautifully, except for one thing.

Say I have three email addresses subscribed to my list. One is from
the domain hotmail.com, one is from the domain gmail.com, and one is
from the domain speakeasy.net.

Now I send an email to the list, and it goes through fine to the Gmail
and Hotmail users. However, it never gets to the Speakeasy user. It
isn't a speakeasy problem since the email doesn't get past my server.

The log entry for the speakeasy one looks like this: 
to=<....speakeasy.net>, relay=virtual, delay=0, status=bounced
(unknown user: "....speakeasy.net")

The log entry for the Gmail one looks like this: 
 to=<...gmail.com>, relay=gsmtp185.google.com[64.233.185.27], delay=0,
status=sent (250 2.0.0 OK 1101868868)

Now I think I'm missing some critical concept here. How did my mail
server know what the gmail relay was? And why doesn't it know the
relay for speakeasy?