Code:
This is the mail system at host mta1.ics-il.net.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<spam.u67m6ydk1@mailbox1.ics-il.net>: connect to
    mailbox1.ics-il.net[10.1.8.5]:25: Connection refused
According to server status in the admin console, both mta1 and mailbox1 are up.

mailbox1 is listening to port 25 on 127.0.0.1, but mailbox2 (which hasn't had any errors) isn't listening to 25 on any IP address.

I used
Code:
zmprov sa zimbraMailHost={mailstore}
to see what accounts are on a given mailstore. I was able to send an e-mail from the outside to accounts on both servers.

Ideas?