Providing mail relay

Question

With apologies for possibly not finding the answer to this elsewhere, I'm looking for advice on setting up my server to be a relay for other people's. On other words...
I specialise in selling to small businesses; they typically run their own mail services on OSX servers attached to ADSL lines with fixed IP. My own set-up is just the same.
My mail server is 'backed up' by two relays provided by my ISP. If I'm down, they hang on to my mail and forward it on when I'm back running again. Some of my customers work with ISPs that don't offer this service and I've been asked if I would offer my server for this purpose. The public dns record for such a customer would have to read something like (I'm classic-keyboard):
mail.customersdomain.com MX 10
mail.classic-keyboard.com MX 20
Obviously I'm not expecting some sort of magical cascade effect whereby my ISP's relays end up backing up my customers as well as me. My server is stable and UPS-protected and I sit next to it and stroke it every now and then. If I'm down, my customers aren't expecting another level of backup.
My questions are these: if I let a customer do that to his zone file, and subsequently my server receives an email intended for him, does my server 'know' what to do by looking up the dns or must I make local configuration changes to support it (and are they specific to each customer)?
And, can my SMTP security remain set to CRAM-MD5 and Kerberos only (I hope so)?
And, will I have to turn off 'Accept SMTP relays only from these hosts and networks' (I hope not)?
And, will my 'Refuse all messages from these hosts and networks' entries apply to relayed mail as well as mail addressed to my own domain (this would be neat)?
And, is this just a really stupid idea that's going to give me more thankless work to do and just get my server labelled as the source of every piece of junk mail my customers receive?
Please don't tell me to go read the manual...
Justin

Answer 1

server for this purpose. The public dns record for
such a customer would have to read something like
(I'm classic-keyboard):
mail.customersdomain.com MX 10
mail.classic-keyboard.com MX 20
Yes.
My questions are these: if I let a customer do that
to his zone file, and subsequently my server receives
an email intended for him, does my server 'know' what
to do by looking up the dns
No.
or must I make local
configuration changes to support it (and are they
specific to each customer)?
Yes and yes. Also, you will not be able to do this with Server Admin. Must edit config files manually.
See: http://www.postfix.org/STANDARDCONFIGURATIONREADME.html#backup
And, can my SMTP security remain set to CRAM-MD5 and
Kerberos only (I hope so)?
Yes. Your security settings are only relevant for sending through your mail server, not for receiving mail from the outside.
And, will I have to turn off 'Accept SMTP relays only
from these hosts and networks' (I hope not)?
No.
And, will my 'Refuse all messages from these hosts
and networks' entries apply to relayed mail as well
as mail addressed to my own domain (this would be
neat)?
Can be either way, depending on how you set things up. Normally, yes.
And, is this just a really stupid idea that's going
to give me more thankless work to do and just get my
server labelled as the source of every piece of junk
mail my customers receive?
If you are not an experienced mail server admin, this will cause only unnecessary headaches and add a lot of responsibility. Up to you to judge whether it makes sense for your business. You may be better of with a simple solution like this: https://www.dyndns.com/services/pricing/index.html#mailhop
Alex