FYI, I finally got the response from your server.  These headers suggest that the bottleneck is indeed somewhere between LISTSERV and the Mirapoint server.

>Received: from [200.23.90.203] by SPIDER.EASE.LSOFT.SE (SMTPL release 1.0v) with TCP; Tue, 19 Feb 2008 21:50:24 +0100
>Received: from *.didage.org.mx (EHLO didage) ([200.23.90.202])
>        by smtp.didage.org.mx (MOS 3.8.3-GA FastPath queued)
>        with ESMTP id AFQ05267;
>        Tue, 19 Feb 2008 14:22:16 -0600 (CST)
>Date:         Tue, 19 Feb 2008 12:16:15 -0600
>From: [log in to unmask]
>Subject: Re: RELEASE

LISTSERV received and acted on my message at 12:16:15, but the Mirapoint server didn't receive it from LISTSERV until 14:22:16 -- over two hours later.  That means the message sat in LISTSERV's spool all that time while LISTSERV tried to get Mirapoint to accept it.  Then Mirapoint inexplicably held it for another half an hour before it forwarded it on to our corporate mail gateway in Sweden.  I suppose the clocks on the machines could be off by a few minutes, but I wouldn't expect them to be off by half an hour or more.

Nathan