hosted email - ptr and smtp banners

We have found ourselves hosting email for a couple of different companies. I was not a planned thing, one company lost their building and equipment, we merged (sort of) with another company that wanted to retain their domain and a third company had their host go under.

All of a sudden we have four email domains coming into our network. It was all very temporary and now it looks like it may be permanent. Right now each domain has an MX record pointing to our firewall. The firewall forwards the request to our Barracuda 300 and then on to the appropriate mail server.

Right now only our domain has a proper ptr record. The other three are using our extra IP addresses. As well the Barracuda responds with an smtp banner that says barracuda.mydomain.com

When we use the diagnostics tools at mxtoolbox.com it has issues with the ptr record not matching and the smtp banner not matching.

I'm curious what would be the normal smtp banner to display in a hosting environment.

For the ptr records I can easily have them created as each domain is using a single IP address. If we continue to do this we will want to consolidate these onto one or two IP addresses, no need to use them all up. How do you deal with ptr records when more than one domain is using an ip address?

I'm guessing there should not be more than one ptr per IP address.

Do you create a single email domain like catchallemail.mydomain.com and change all the mx records to that? That gives just one ptr record and the barracuda should then sort the email by domain.

Thanks for any info