You'll have to set up multiple listeners (daemons) (one per ip) predefined with the correct helo/ehlo setting. Smtp has no knowledge of the host used for the connection unfortunately
Edit:
You're asking about outbound. I wrote a tutorial that does this. It's basically sending from multiple IPs and with each IP you can change the name it uses. http://iredmail.org/docs/send.out.email … esses.html
The smtp_helo_name is the parameter you'll want to make sure matches your ptr rDNS

Some mail servers do not like the rDNS not matching as it often implies spam so I'd do it just in case. Ignore the top piece I wrote, just read the edit bit please

its right but they need different identifiers
name them like {domain without extension}-smtp instead of sample-smtp
and in sdd_transport.pcre you'll need to use the correct one instead of all of them being sample-smtp

Yes
don't forget this line in main.cf though
sender_dependent_default_transport_maps = pcre:/etc/postfix/sdd_transport.pcre

This is for outbound. MXToolBox tests inbound, the setup for that would be additional to have the Banner change depending on which IP you connect through

AH! Thank you, I think I understand now.

So I have achieved what I hoped for but you mean the mxtoolbox SMTP test is the wrong tool to test the changes you suggested? I must have confused us both by telling you what I wanted but also mentioning the mxtoolbox test, I think you edited an earlier post when you realised what I meant was different to what I was saying.

Ok, I checked the properties of an email I had sent to myself at another address. I now realise that although it looks as if the server is giving the correct response, it still doesn't seem right to me but not due to iRedMail. My ISP assigned me some static IPs (lets say 111.222.333.100 to 111.222.333.200), I have created virtual IPs on my firewall to listen for the additional IP address traffic and NAT'd rules to the internal servers, and I have mapped hostnames to PTR records, but of course whatever DNS records I set up all of my outgoing emails still appear to come from 111.222.333.100, the first public IP address.

I suppose I need to force the outgoing traffic out of each domain's assigned IP address, but as I say not a job for iRedMail. Thank you again for all your help.

I still must be missing something in my understanding then. My network is behind a NAT firewall, and all hosts behind it have internal IP addresses. I do not see how "mail.domain.com" can claim or bind to 111.222.333.100 when its IP address is 192.168.0.100, it has no ownership of the external IP.

Now that I am explaining it this seems obvious to me and I apologise for not being clear, I think I need to configure my firewall to directly present 111.222.333.100 to an internal host, then I can see how your advice would come in to play.

Thanks again, I had just begun researching how to achieve this and you have narrowed down my choices (and suggested the method that seemed most secure). I do appreciate all your help and patience.