I have the following configured:
mail.firstdomain.com - 22.214.171.124 - This domain sends out a lot of email (our newsletter to clients), and thus I keep the two separated.
mail.seconddomain.com - 126.96.36.199 - This domain is were we send out email from our app (registrations, notifications, etc...)
mail.mypersonaldomain.com - 188.8.131.52 - This is my personal email hosted by Google/GMail
I have some email that are sending from mail.secondomain.com being rejected:
Could not deliver message to the following recipient(s): Failed Recipient: firstname.lastname@example.org Reason: Remote host said: 554 mail server permanently rejected message (#5.3.0)
In the header:
Received: from 184.108.40.206 [220.127.116.11] by mail.mypersonaldomain.com with SMTP; (date and time)
How is this possible?
18.104.22.168 is not involved in the email sending. I assume this is from the recipients server associating the email from 22.214.171.124 with the other IP (126.96.36.199) and further, with my personal email server (mail.personaldomain).
I checked all the configuration, and I don't see anything other than mail.seconddomain.com being used. 188.8.131.52 and 184.108.40.206 are even in separate data centers.