Another possibility: tarpitting or throttling on the SMTP host. Be sure your LISTSERV's IP has been exempted from anti-spam measures like that on your SMTP host.

On Feb 11, 2013, at 3:37 PM, Valdis Kletnieks <[log in to unmask]> wrote:

> On Mon, 11 Feb 2013 15:17:07 -0500, Liam Kelly said:
> 
>> As to why your SMTP server is taking so long, you'll need to run it down 
>> from that end.  Some of the usual causes are things like inefficient 
>> anti-virus and/or spam scanning of every message (my first guess), a disk 
>> I/O bottleneck on the mail spool disk, inbound mail throttling, or 
>> inefficient directory lookups for addresses to route locally.  Whatever 
>> the case may be, you aren't going to be able to figure it out by looking 
>> only at LISTSERV's end of things.  You need to go to the mail server.
> 
> Even today in 2013, an amazing percent of SMTP servers have broken DNS
> setups that cause a lot of waits for DNS lookup timeouts.  Just a thought...
> 

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