Relay a domain to a second mail server

[Silent Ninja]

First: This is my actual situation...

An ISP is blocking our webserver ip range on their mailservers, not sure why, and also they refuse to unlock us; anyhow, we still need to send emails to them.

We've figured out that all the emails going to @ispaddress.com should pass via another cpanel webserver of ours via relay (this one isn't blocked) and thus the mails will be delivered correctly.

I've seen something about ACL's and relay_hosts or transports, but I haven't been able to find a config that actually works on cPanel and that was easy for me to edit it so that just @ispaddress.com emails get's routed by the another smtp server and all the other mails get's delivered normally.

Could you help me with this setup ?

[Silent Ninja]

Fixed with this guide:
Exim with a remote SMTP server - ArchWiki

Not as simple as it looks but kinda
On the first editable block on Advanced Exim Editors, add this:

domainlist blocked_me_domains = lsearch;/etc/blockedmedomains

Obviously you'll have to put all your domains blocked on "/etc/blockedmedomains" (one per line on the file).

Then add this after the editable box right after the "# Demo Safety Router":

to_myothersmtp:
  driver = manualroute
  domains = +blocked_me_domains
  transport = remote_smtp
  route_list = * your.smtphostname.here

Obviously editting where it says: "your.smtphostname.here" and allowing to relay on that smtp (you may whitelist yourself to make it faster).