Recipient List Cut Off in Outlook Express

I'm seeing a number of emails sent to a qmail server from Outlook
Express with the recipient list getting cut off at the 41st or 42nd
email address. The subject line ends up being blank, and the remainder
of the recipients (along with all the headers) are at the beginning of
the body of the email.

I've looked at my qmail settings, and they're all good--I'm only seeing
this from users in OE and some using Outlook XP (other mail clients
work fine, and sending to the same number of email addresses in the BCC
field goes through fine). Anyone have an idea of a fix for this?