Apparently it happens due to the checks applied for sender_address_domain for remote_smtp transport in exim.conf.
In case of the forwarder, the variable sender_address_domain used for the IP lookup in "remote_smtp" is useless due to the fact the forwarded messages preserve the original sender as $sender_address_domain.
Consider it as
1@test.tld - locally served
2@anothertest.tld - gets emails forwarded from
1@test.tld
If you send an email from
external@gmail.com or w/e to
1@test.tld
the email is forwarder to
2@anothertest.tld
as from
external@gmail.com
On the remote_smtp tranport time, the sender address
external@gmail.com is checked for the records in /etc/mailips and sure thing, not found...
You can always debug the delivery using the command like
exim -d -bt
1@test.tld
@ cPanel guys - are you planning to add a check "if it's forwarded/exists in valiases - use local domain record from mailips for the interface " ?
I bet the most users would be interested to see it...
Just 2 cents...