Reverse DNS lookup with dual domain names

[irbk]

My company was just recently purchased, so now I have the joy of trying to have everyone having dual e-mail addresses. user@oldcompany.com and user@newcompany.com.
I've got all my DNS records set up correctly (I believe).
However, I've had occasional issues where e-mail from sources outside of my network just never show up. No error messages, no nothing. The weird thing is that someone could send e-mail to user1@newcompany.com & user2@newcompany.com. One may see the e-mail, while the other one never will. I wonder if it's possibily DNS or reverse DNS?
For the original domain oldcompany.com
oldcompany.com has an MX to mail.oldcompany.com
mail.oldcompany.com has an A record to <$my_ip>
for the new company it's the same
newcompany.com has an MX to mail.newcompany.com
mail.newcompany.com has an A record to <$my_ip, which is the same IP as mail.oldcompany.com>

Now if you do a reverse lookup on $my_ip, it resolves only to oldcompany.com. This is why I'm wondering if reverse DNS might be the issue. So how do I go about setting up my MX and DNS records correctly so that anyone that anyone running a reverse DNS lookup will be able to send us e-mail regardless of if the e-mail is sent to @oldcompany.com or @newcompany.com?

Thanks in advance!

 

[elgrandeperro]

Since PTR records can only return a single name, I would do it this way:

newcompany:
newcompany.com. IN MX 10 mail.newcompany.com.
mail.newcompany.com. IN A a.b.c.d

oldcompany:
oldcompany.com. IN MX 10 mail.newcompany.com.
mail.oldcompany.com. IN A a.b.c.d

where the PTR fox a.b.c.d returns mail.newcompany.com.

 

[irbk]

Can I have 2 MX records at 10? I didn't think you could have them at the same amount. Or is that only for the same domain name?

 

[elgrandeperro]

We are talking separate domains (new and old) so each domain with only 1 mail gateway, you would have one entry per zone (if using bind/named) per file.

like newcompany.db:

SOA (etc.)..
@ IN MX 10 mail.newcompany.com.
mail IN A A.B.C.D

and in oldcompany.db:

SOA (etc.)...
@ IN MX 10 mail.newcompany.com.
mail IN A.B.C.D (not needed, but added for compatibility for other things).

If you had MORE than one gateway, you certainly can put them at the same precedence number in the SAME db file:

SOA (etc.)..
@ IN MX 0 firstgateway
@ IN MX 0 secondgateway

The MTA would try one then the other before queueing. The ordering is per domain, so the MX numbers are all relative within the domain, it just specifies the order WITHIN the domain the mail servers are tried.