Cron – Exim – Force Messages to domain before retry time

Is my plan feasible?

No, not really. The problem is (almost) anything in an email and email header can be changed and spoofed. Also anyone in your company can send email through any other server they like to (unless you block all their internet access, oh wait, you can't, they still have their smartphones etc.), seemingly coming from your company. If an employee in your company wants to avoid the scrutiny of the PHB they just use an outside email server (MTA).

You don't even have to worry about deliverability, say if the rDNS or EHLO greeting doesn't match the return-path. Just make the return-path's domain be the same as the outgoing MTA, but change the From: address to be from your company and almost everyone thinks it IS from your company.

Will there be issues that I have overlooked?

Yes, always. It is a pointless exercise and it is trivial to circumvent. To avoid yourself and your co-workers any headaches you need to explain this to your PHB and convince her or him that it's the wrong way to go. There are better ways to approach this. But those lie more in company policy, educating and hiring the right employees, and specific regulations than in extreme (and easy to avoid) measures limiting one's freedom. These kind of draconian measures have a knack of biting you back in the long run, almost without exception.

Does it have the danger of being treated as spam by the other mailservers since I'll be messing up the headers?

Not likely. Anything in the headers can be spoofed and changed (see above). That's pretty much the standard, MTAs add and change things in the headers all the time. Whether you do it as a human, or create a script to change things, the MTA doesn't really care. As long as it is validated and accepted by the MTA it should not have much more of a chance to be marked as spam (within reason of course).

Anyways, I don't want to think any more about this truly horrendous idea. :-(

Well, that was easier than expected.

I noticed my exim settings file (/etc/exim/exim.conf) contained this commented out line:

#helo_data = $sender_address_domain

Uncommenting it out caused the HELO / from to become the right domain, just missing the mail. prefix.

So what worked for me was:

(1) Under remote_smtp: in my exim config file, which on my system was located at /etc/exim/exim.conf, adding the following line:

helo_data = "mail.${sender_address_domain}"

(2) Restarting exim (on my system, that was service exim restart)

(3) That's all

After making this change, services like Mail Tester find no problems, and this section of my mail headers look normal:

Received: from mail.domain-c.com (mail.domain-c.com. [33.33.33.33])

The hostname does still appear later on in the email headers; I've asked a separate question about that.