Exchange Server 2003 and 4.4.7 time limit expirations

One of my consulting clients contacted about some email problems they were having. The staff was receiving an increasing amount of non-deliverable report emails when sending to valid addresses of clients they communicate with on a daily basis. I checked the messages they were receiving back and most had the following reason listed: Could not deliver the message in the time limit specified. Please retry or contact your administrator. The problem was inconsistent, of course. For example, there were 20+ messages waiting to go to Hotmail, listed as either "retrying" or "pending" in the Exchange System Manager Queue for the server, but once in a while a message would go through. They were able to receive emails from outside and if I sent messages to Gmail for example they'd come and go without any issues. I enabled the SMTP diagnostic logging, I enabled the SMTP server to write the extra logging information to try and determine the problem, but all without any indication of why the connection was being dropped. The logs seem to indicate that the connection happened, the mail from and mail to commands were successful as well as the transmission of the email, but then the connection was dropped. Since it seemed that the majority of the failed messages were to Hotmail, I followed their guide for trouble-shooting at http://mail.live.com/mail/troubleshooting.aspx. I also submitted a request to open a ticket with Hotmail (haven't heard anything yet). I used the Microsoft Sender ID wizard (http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx) to generate a SPF record for the DNS, but nothing seemed to correct the problem. Finally I found a comment in a forum posting for the anti-virus software running on the server at http://supportforums.sunbeltsoftware.com/messageview.aspx?catid=27&threadid=2233. This post mentioned to make sure, when running VIPRE on the Exchange Server, to have the client email protection disabled as it can't handle the load of the SMTP connections on an Exchange Server. After disabling that option, then restarting the SMTP service, all of the mail queues were emptied and the mail was flowing once again.