Why do emails sent to comcast silently time out? ( 4.4.1 error )

We provide web hosting for a handful of small businesses, and recently one of them has started having difficulty contacting even their own personal @comcast.net addresses. They don't receive any error messages, up until our mail server gives up trying to send the message.

We checked with the Comcast RBL, and found we weren't listed - nor would we have any reason to be, to the best of my knowledge.

The message our clients receive is:

From: MAILER-DAEMON@studioblueprint.net
[mailto:MAILER-DAEMON@studioblueprint.net]
Sent: Friday, May 08, 2009 6:20 PM
To: *****@thelanguagefactor.com
Subject: failure notice

Hi. This is the qmail-send program at studioblueprint.net.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.

<*******@comcast.net>:
Sorry, I wasn't able to establish an SMTP connection. (#4.4.1)
I'm not going to try again; this message has been in the queue too long.

Any help you could provide would be fantastic.
-Andrew