"LISTSERV site administrators' forum" <[log in to unmask]> 
wrote on 06/24/2005 11:31:22 AM:

> We've recently put a graylisting spam filter at the edge of our 
> campus network, but at my request the Listserv was left outside the 
> filter because I was concerned that graylist processing of all the 
> traffic coming in to the server would introduce delays that would 
> somehow harm Listserv.
> 
> Has anyone out there put their Listserv behind a graylisting spam 
> filter? The Listserv server has been getting hit pretty hard by 
> phishing scams lately, which seem to be coming in under Listserv's 
> net-wide spam radar. Currently our graylist is catching these spams 
> but a few of them always seem to leak in through Listserv. I'd like 
> to have Listserv moved behind the graylist filter if it won't harm 
> the operations of the server. We are a backbone server running in 
> networked mode.

Our spam filter does greylisting.  I do not have the listserv behind it 
though.  I didn't want to block any legit mail from the lists.  Spam on 
the lists has not been a complaint here.

Any non-braindead mail server should not have an issue with greylisting. 
There are a few braindead servers out there.  Groupwise, at least some 
versions, have a problem depending on when the tempfail is issued.  If it 
is issued after the recipient, and every RCPT TO: is tempfailed, it would 
return the message as undeliverable.  CanIt (http://canit.ca) offers a 
setting to delay the tempfail until after the DATA phase as a work around.

I ran into one other instance with a mail security system that had 
trouble, but it was fixed quickly once it was reported to the vendor.

Can you set up an alternate name in DNS for your listserv and route that 
through the greylisting spam filter to test?