Question about virus/spam filtering for customers with mail servers

[Richard Kuhns]

I'm hoping someone will be willing to share a better way to handle this.

We offer virus/spam filtering for customers with their own mail servers. 
We're currently implementing this by configuring the customer's firewall 
to only accept smtp connections from our servers (all running sendmail 
under FreeBSD 4), and the customer's MX records point to their server 
first and our server(s) second and third. In most cases this works just 
fine -- attempts by a mail server to deliver mail directly to the 
customer fail, the mail server tries the secondary MX site (us), we 
accept and filter the message and deliver it to the customer (or not).

Sometimes, though, there's a very long delay for messages to be 
delivered - up to several days. In each case I've been able to track 
down, it's been a Microsoft Exchange 2000 server that has issues with 
sending messages to the secondary mail server.

Does anyone have a good way we could use to list our server as the 
primary, and then forward the messages? I've been going through the bat 
book, but the indexing leaves a little to be desired and I haven't found 
anything that looks applicable yet.

Any suggestions would be greatly appreciated. Thanks in advance...
	- Rich
-- 
Richard Kuhns                   Wintek Corporation
E-mail: rjk at wintek.com          427 N 6th Street
Tel: +1 (765) 742-8428          Lafayette, IN 47901-1126
Fax: +1 (765) 742-0646          United States of America