We are currently trying to track down an email that gets delayed between our servers. Here our the timings in the mail headers:

Received: (qmail 293 invoked from network); 20 Mar 2012 05:39:49 +0000
Received: from HOST_1 (000.000.000.000) by OUR_HOST with SMTP; 20 Mar 2012 05:39:46 +0000
Received: from mail1.issociate.net ([212.127.35.10]) by HOST_1; Tue, 20 Mar 2012 05:29:37
Received: from HOST_3 (?????????.de [000.000.000.000]) by mail2.issociate.net with SMTP;Mon, 19 Mar 2012 15:57:57
Date: Mon, 19 Mar 2012 15:57:36

The emails are coming from germany - am I right in thinking that issociate.net is keeping the mail on their servers for a long time?
When we send outgoing mail to them - they get it fairly quickly. We have whitelisted etc. as much as we could.
Any help much appreciated.

Also... HOST_1 is our relay server, even without it there is still a big delay.

It is definitely showing a huge delay between issociate.net receiving the message and HOST_1 accepting it. But it doesn't show if issociate.net tries to send it in the meantime and HOST_1 rejects it or can't accept it for any reason.

The maillogs on HOST_1 would show a connection if it did (and maybe more info). If no connection attempt then it is all outside your control and nothing you can do.

Mailservers often introduce big delays like this when they are swamped with spam.

it could be greylisting too.
if for example they send the mail from a different mailserver every time they try to resend the mail it would create new greylist entry and wait for the second one.
do you have greylist functionality enabled? if so try to whitelist it for that host range like *.issociate.net.