[Pkg-exim4-users] SMTP Problems

[Maarten Bezemer]

On Fri, 13 Jul 2012, Robert Steinmetz AIA wrote:

> I have recently become aware of a problem sending mail to one of my 
> customers, it seems some mail silently fails to be delivered. I have tried 
> telneting into their smtp server and all connections fail with time outs.

Telnet into their smtp server might not be the correct thing to do... 
check the domain's MX record(s) and telnet into those servers on port 25.
If those reject / timeout your connection, make sure you are not yourself 
behind some kind of firewall that blocks outgoing connections to smtp 
ports other than your own ISP's. (This is quite common on domestic DSL / 
Cable lines!)

Try to connect some other servers over port 25, and see what it does. If 
those connections also time out, it's probably something on your end.

Next thing to check if your own IP address has been listed in DNS 
blacklists used by many organisations to simply block you from connecting 
to their servers. You mentioned your customer is a large company, so they 
probably have measures like these in place to reduce spam, phishing, 
malware, etc..
Being in a domestic DSL/Cable IP-range places your IP address on several 
blacklists, so that could explain the server rejecting your connection.


> Any idea how I can troubleshoot this issue? My customer is a very large 
> company and seem disinclined to admit the problem is on their end.

Check MX records for the right server name to connect to.

Connect to those servers, not only from your own IP address but also from 
other IP addresses (e.g. home/office, another customer site).

Try sending an email to the address from gmail/hotmail/whatever and see if 
that works. Request the customer to return the email headers.

If mail from gmail/hotmail/... is accepted, it's definitely not their 
server that's doing something wrong. Or, more accurately, it may be acting 
a bit overzealous by blocking your server.

However, legitimate mail should never be dropped silently. After a few 
hours / days (depending on the configuration) a delivery status 
notification is sent to the sender. Possibly including reasons for the 
delay or failure. Exim's mainlog could also give some more info about the 
reason an email is not (yet) delivered.


HTH,
Maarten