To enable different helo commands on multiple IPs we need to utilize the Exim router and transport settings, these are available in Exim 4+.
An example router:
driver = dnslookup
senders = *@mydomain.com, *@*.mydomain.com
transport = mytransport
The senders option is a CSV list of addresses to match in the From header. These can contain regex which makes matching whole domains really easy.
The transport option is a reference to your custom transport (see below).
An example transport is:
driver = smtp
interface = 192.168.0.1
helo_data = mail.mydomain.com
The interface setting is the IP address this transport should be used on and the helo_data is the command to send.
This makes Exim an awesome platform for shared mail servers. It is very simple to offer whitelisting and protection against spammers.