How to make mailcow ignore the MX record of a sender dependent transport

sfgreenwood

I am setting up a mailcow server to use an existing sender dependent transport prior to migration from an existing system. The transport host has, for historical reasons, an MX record of its own. Mailcow is looking up the MX record of this transport host and attempting to use it as the transport, which fails because the transport is configured to use port 2525 and the MX record isn’t. Is there a way to make mailcow use the transport host instead of the MX record?

esackbauer

It is not enough to just create the sender dependent transport, you must also assign it do the domains you configured.
Otherwise it will keep looking for the MX records.

sfgreenwood

It is set up with a domain. I run a few mail services including mailcow ones and I know how to set up sender dependent transports. The issue here is that this transport host has an MX record and postfix is substituting the hostname of the MX record for the hostname of the transport.
Reading around, I can see this is because postfix will prefer an MX record if one exists. The MX record is used for bounces in a multi-server system so I probably can’t delete it at the moment (it was set up before my time).
I have added a wildcard transport to send all mail to the relay host, as will be happening in production, and that hasn’t worked.
The Test function does work for the sender dependent transport and for the transport, and I have tested this with another email address that I own and it delivers the mail directly through the relay host. I assume this uses a script rather than postfix.
I think I need to be able to override the MX lookup in some way so I think what I need to know is the best way to do that in Mailcow.

esackbauer

That is strange, because if I would in your case send e.g. to gmail.com, gmail.com has a MX record, so mailcow would try to send directly using the MX? and ignore the sender dependent transport? Which is obviously not the case with my server, as mails are flowing out to gmail via my relay host configured as sender dependent transport.

For clarity, can you bring examples with A and MX records how your landscape is configured?
Have you tried to configure the sender dependent transport with IP address instead?

sfgreenwood

Ah, that’s a good point, I’ll try with the IP address.

To illustrate the problem though, this is the host lookup of the relay host:

host outboundary.lawyersonline.co.uk outboundary.lawyersonline.co.uk has address 78.31.107.192 outboundary.lawyersonline.co.uk mail is handled by 50 securereceive.lawyersonline.co.uk.

and this is how a mail is being processed in mailcow:

87F13A2945: to=<sfgreenwood@gmail.com>, relay=none, delay=502, delays=471/0.06/30/0, dsn=4.4.1, status=deferred (connect to securereceive.lawyersonline.co.uk[35.246.35.152]:2525: Connection timed out)

At no point am I adding securereceive.lawyersonline.co.uk anywhere. This would also be the MX record of the lawyersonline.co.uk domain. When I first added the relay host I thought I caught a log entry that referred to checking the MX record but I can’t find it now.

esackbauer

Why you have MX records for such hostnames like “outboundary” or “securereceive” at all? That is not needed. You need that only for the actual mail domain which I guess is “lawyersonline.co.uk”

sfgreenwood

Again, historical reasons - it’s a legacy multi-host system that I am migrating to Mailcow so there is a lot of junk in DNS that probably doesn’t need to be there.

esackbauer

Have you tried to use the IP address instead in the sender dependent transport?
Are there any other manually configured rules in the extra.cf files etc?
What you could also try is give the relayhost a new A record with lets say “relay.lawyersonline.co.uk”. Put this into the sender dependent transport. As no MX record for that exists, it should work.
You dont have a (non-sender dependent) transport rule for outboundary.lawyersonline.co.uk, hopefully?

sfgreenwood

I did have a non-sender dependent transport rule :/
Let me try sending a mail again.

Yes, that worked. Stupid of me.

As I say, it’s an old system and has been running in different forms for nearly 30 years so it needs tidying up.

Just to summarise it in the unlikely event that anyone else has a system like this:

Postfix looks up the MX record of a host and attempts to use that, so that is what it did here.

Using the IP address of the relay host bypasses the lookup and uses the relay host directly.

esackbauer

sfgreenwood Postfix looks up the MX record of a host and attempts to use that, so that is what it did here.

Could it be that the transport map (not the sender dependent one) was the culprit?
As it says in the configuration screen, the first 2 lines of the transport map:

• A transport map entry overrules a sender-dependent transport map.
• MX-based transports are preferably used.

sfgreenwood

I added the transport map when I couldn’t work out why the sender dependent transport wasn’t working, so it had the same problem of looking up the MX record, so thanks for the reminder.
The specific problem is that the mail system I’m migrating does non-standard things, which can probably be over-ridden with postfix configuration, but using the IP address of the relay host works as well,