PTR record and dyndns

To answer your new question first: No, PTRs don't tell you anything about the sender's domain. See below for the explanation.

Now back to your original question:

Receiving mail servers check none, one, many or all of the following:

1. Is the HELO name equal to the hostname (A record)?
2. Is PTR of the IP equal to the A record of the hostname (hostname ==(PTR)==> IP ==(A)==> hostname)?
3. Is the IP part of the provided SPF record?
4. Does the sender's domain have at least one MX record? Which doesn't have to match IP/hostname.

Receiving mail servers that check if the sending server is also the MX server are badly configured and should be eliminated from the Internet.

Edit: The PTR does absolutely prove nothing about the email domain. It is never meant to prove this. There are thousands of domains hosted at Google, Amazon, AOL and others. But none of them match the hostname or PTR of Google, Amazon, AOL and others. They all have the servernames of the providers. And there is nothing bad about that.

The PTR only proves the identity of the server but not the identity of the hosted domains. Point.

2nd Edit: A good example for a working environment would be

• HELO = mail.example.com
• hostname = mail.example.com
• A record of mail.example.com = 172.20.25.25
• PTR of 172.20.25.25 = mail.example.com
• Domains hosted on this server/system = example.com, *.example.com, *.example.net, *.example.org, mycompany.invalid and many many more.
• SPF records of hosted domains (optionally) = v=spf1 a:mail.example.com -all
• MX record of hosted domains can be anything. E.g. mx1.example.com, mx2.example.com, mailfilter.anti-spam-corp.invalid, mail.example.com, postini.google.invalid, ...

It does not matter what domain you're sending e-mail for. Explicitly, it does not matter what your employee's e-mail addresses are, it does not matter if they're joe@eample.com. I need to make that point 100% clear because everyone assumes it has something to do with this.

Now, your e-mail server has a hostname. For many companies it something like mail.example.com. This is what matters in regards to the PTR record. DNS for the mail server must match forward, backward, and with regards to the SMTP communications.

That means:

1. Your forward host name must resolve to the correct IP.
   mail.example.com must resolve to 1.1.1.1
2. Your reverse pointer must resolve to the correct host name.
   1.1.1.1 must point to mail.example.com
3. When your server connects to another server, it must connect from IP 1.1.1.1 and announce itself as mail.example.com (the HELO or EHLO message).

If you have more than one server they each get their own host name (eg mail1.example.com and mail2.example.com), and the above has to be true for each server.