I concur with Melissa.  It's likely a remote email host is complaining that you sent them too many messages (or in my case a message with too many recipients) to that site.  I tried to appeal to the remote email administrator without success--"they didn't have a problem".  I finally reconfigured my email server so that outbound messages to that site were broken down into 5 recipients per message copy.  That was the solution to my problem - your mileage may vary.
-Rich

> -----Original Message-----
> From: LISTSERV site administrators' forum [mailto:LSTSRV-
> [log in to unmask]] On Behalf Of Kenny, Melissa S
> Sent: Monday, September 13, 2010 12:34 PM
> To: [log in to unmask]
> Subject: Re: host connection threshold exceeded for...
>
> I think this problem is from your enterprise SMTP servers.  They're
> objecting to Listserv sending too many messages in one connection.  You'll
> need to contact whomever runs those servers and ask them to "whitelist"
> your Listserv server.
>
> Melissa
>
> ________________________________________
> From: LISTSERV site administrators' forum [[log in to unmask]]
> On Behalf Of Rick Scheller [[log in to unmask]]
> Sent: Monday, September 13, 2010 11:08 AM
> To: [log in to unmask]
> Subject: [LSTSRV-L] host connection threshold exceeded for...
>
> We are using ListServ 15.5 and are relaying out to a group of 5 smtp
> server at our enterprise level.  The logs on these servers are filled
> with:
>
> 09-13-2010.00:10:04 SMTPserver-522: 1963484 host connection threshold
> exceeded for xxx.xxx.xxxxx.xx.xx
> 09-13-2010.00:10:04 SMTPserver-522: 1963485 host connection threshold
> exceeded for xxx.xxx.xxxxx.xx.xx
> 09-13-2010.00:10:05 SMTPserver-522: 1963486 host connection threshold
> exceeded for xxx.xxx.xxxxx.xx.xx
>
> the logs on our side look like:
>
> 14 Jun 2010 09:26:08 <421 Service not available, closing transmission
> channel
>
> like 400,000 times a day.
>
> The Listserv logs look normal.  All the list are working fine.
>
> Has anyone  run into this before?
>
> Thanks in advance,
>
> Rick