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I'm not familiar with Exim but from my experience these type of delays (a couple of hours) are usually caused by problems with DNS queries regardless of the type of e-mail server. (I'm assuming that server 2 is always up since it is under your control).
You may find utilities ``dig'' or ``nslookup'' useful to try to narrow down which DNS queries are being delayed. My bet would be that the delays are caused by reverse lookup DNS queries on server 1. But don't let that limit your investigation it is just a guess.
If the delays are not caused by DNS queries but by SMTP related problems you will have to get access to the content of the SMTP logs, somehow.
you make a fair point but I still suspect DNS. I'm assuming that server 1 is configured to RELAY e-mails addressed to the server 2 domain. As I said, I'm not familiar with Exim specifically but I suspect DNS exactly because your servers can receive e-mails without a problem. From my experience with other e-mail servers I come to the conclusion as follows:
You experience delays, not failures. That means that server 1, when it attempts to RELAY an e-mail to server 2, is seeing errors that it considers temporary and places the e-mails on the deferral queue to be sent later rather than rejecting them outright.
Temporary errors can be roughly divided into three types:
1) temporary SMTP errors (e.g. mailbox full)
2) SMTP connection dropped or timed out, for example, because server 2 is too busy
3) temporary inability to make an SMTP connection
I consider 1) and 2) less likely because you would probably already know about those problems even though it would still be very useful for you to have access to the SMTP logs to be able to rule out those problems (and many others) with confidence.
That makes problems of type 3) the most likely reasons for your delays. Type 3) problems could also be caused by some very severe network problems but, again, I'm sure you would be already aware of them.
When an e-mail server is asked to RELAY an e-mail, it does more than just receive it and send it. Since RELAY is frequently abused by spammers the e-mail server performs various checks on the originating domain and that involves making DNS queries (it may also perform a black list check but that would result in permanent failures not delays).
Those DNS queries may be failing because of DNS misconfiguration, overloaded DNS servers or because the e-mail server is unable to handle some results of those queries.
If the query failures are due to some internal problem with the e-mail server (some servers have a problem handling results of the ANY queries) you will, again, need access to the SMTP logs to prove it.
If the query failures are due to the DNS server themselves you should be able to duplicate it using the ``dig'' or ``nslookup'' tools running on server 1.