If you want everyone on the server to send out on the same IP, just add the following to
# nano /etc/mailips: *: xxx.xxx.xxx.xxx
Then add the IP and it’s matching PTR to /etc/mail_reverse_dns:
# nano /etc/mail_reverse_dns # xxx.xxx.xxx.xxx hostname.tld
This will tell Exim to use that IP for any sender on the server.
Restart exim
# service exim restart