My current smtp server setup is like this:
Internet -> server1:25 (haraka1 smtp server) -> server1:2525 (haraka2 smtp server) -> server2:25 (Microsoft Exchange)
server2:25 (Microsoft Exchange) -> ISP's SMTP server (smarthost in microsoft language)
Now I would like to add a middle smtp server for outgoing emails too:
server2:25 (Microsoft Exchange) -> server1:25259 (haraka smtp server) -> ISP's SMTP server
My question is, will it cause any problem for using a nonstandard port (25259), for outgoing emails?
From exchange point of view the haraka smtp server (server1:25259) is a smarthost.
I want to achieve the following things with this new setup:
- I can sign all outgoing emails with DKIM
- I can send part of the emails through my ISP's smtp server (normal email traffic), and through sendgrid (transactional emails)
- rate limiting, and spam sending prevention (ie. 200emails/day maximum per user, except haraka receives a special email)
Sending emails from a client is like this:
client (from the internet) -> server1:587 (haraka3) -> server2:25 (Exchange) client (within the same lan) -> server:25 (Exchange)
I was forced to put haraka smtp servers in front of Exchange servers, because lately I got many brute force attacks from the internet (about 20.000/day smtp auth tries). And haraka is really simple to modify for my needs (ssl enforcing, rate limit, ehlo checking, etc, etc).
The above email system is rather small, I have about 50 users and about 100-500 email/day outgoing traffic.