I recently upgraded to 5.0.5 (instigated by another issue) and since then one of our users is experiencing some oddities. Among them is all received e-mail is in text format, despite having all settings for HTML checked that we can find in the personal settings area and his account in the management site.

The more important issue that has cropped up is all his mail to internal distribution lists are getting returned:
From: "Mail Delivery System" <MAILER-DAEMON@mail.domain.com>
To: user@domain.com
Sent: Friday, May 23, 2008 10:26:13 AM GMT -06:00 US/Canada Central
Subject: Undelivered Mail Returned to Sender

This is the mail system at host mail.domain.com.

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

<list@domain.com>: domain.com
Myself and several others have no problems sending to this same distribution list. Can anyone give me some pointers on how to resolve this? I'm hoping it's something simple I just overlooked. Thanks.

Edit: I've verified there are no filters present and I've looked for any obvious (I think) settings in his account on the management site.