Perplexed by SMTP Current Sessions queue

Recently had a stop pending on one of the SBS 2k3 servers I
administer.

Checked the current sessions queue and there were over 50 sessions all
with the same IP and domain - each had thousands of seconds in connect
time. The connections were via our offsite Backup mail server.

I terminated all sessions and immediately checked this server for open
relay, both in config and via one of the free Open Relay checking
services. Not Open relay.

Immediately scanned server for trojans and viruses etc. No trojans or
viruses.

Manually scanned each network workstation for trojans and viruses. No
trojans or viruses.

Exchange is connection filtered via subscriptions to Spamhaus,
Spamhaus Zen and Spamcop RBLs.

I also run Recipient filtering and IMF - overall a very effective spam
strategy.

Since denying connections to the backup mail server there have been a
number of incidents of persistent connections in the current sessions
queue, each time I manually add these to the deny connection config,
then stop and restart the SMTP connection.

This past holiday weekend (July 4th) the queue accumulated 6
connections ranging from connect time of 44,865 seconds to 4,800.

What more can I do here to stop these connections. Do I just
disregard?

I manage 3 other SBS 2k3 servers (different companies, domains and
locations) all with the same configuration - none have this problem.

.

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