[Exim] SMTP Command Timeouts - from certain senders only

I receive regular log reports of an SMTP Command Timeout only from certain
users.

the log snip below shows that one user can get through right away and
another get's hung up.

I also have seen in the log where a single message from one user gets hung
up when the next one, seconds later gets through, then the first connection
times out.

I have suspected the SMTP verify command and disabled that, I have disabled
RBL and set smtp_receive_timeout to 15 minutes and smtp_command_timeout to
20 minutes to try to decipher the source of the message.

It looks like the remote machine tries to establish the session then goes
to sleep. The really determined senders will get usually get through
sometime before their 5 day error message gets rejected, others do not.

Some messages from one sender going through the same host will get blocked,
some will get through, then the first messages get rejected back to the
sender by their MTA as a bad address.

I can't say for certain but this also appears to involve mainly qmail MTAs
on the other end.

Is there a qmail vs exim ideosyncracy that I need to be taking into account.

It should also be noted that the box1.mpowercom.net address is my backup
mailserver.
About 1/2 of the messages I get on a given day will go through it.

Any pointers would be much appreciated.

TIA

<snip>
2001-12-29 04:50:18 SMTP connection from keiko.ebay.com [216.33.156.139]
2001-12-29 04:53:53 SMTP connection from box1.mpowercom.net [208.57.0.10]
2001-12-29 04:53:53 16KIzp-0000Bu-00 <= hebay@???
H=box1.mpowercom.net (mgci.com) [208.57.0.10] P=smtp S=1765
id=200112290912.fBT9CRa24020@???
2001-12-29 04:53:53 SMTP connection from box1.mpowercom.net (mgci.com)
[208.57.0.10] closed by QUIT
2001-12-29 04:53:53 16KIzp-0000Bu-00 => gene_b <gene@???>
D=localuser T=local_delivery
2001-12-29 04:53:53 16KIzp-0000Bu-00 Completed
2001-12-29 05:05:18 SMTP command timeout on connection from keiko.ebay.com
[216.33.156.139]
<snip>