of course you can have 5 A records with the same name.
"The reason I have separate IPs was to resovle the problem of not being able to send emails from a domain where the PTR record of the ip address did not match the sending domain name."
ok, stop right here then.
Single IP address both forward and reverse resolvable will be exactly what you need.
Any server that rejects because your users domain does not match the sending servers hostname domain is probably rejecting the worlds email anyways.
Think about gmail, messagelabs, postini, Mimecast, Microsoft and all the others out there providing mail services. NONE of them pretend to be their cleints. ALL of them send from a server that is exactly what it says it is.
