The name provided as a response to HELO/EHLO should be resolvable to the sending server's IP address (A/CNAME record) and it's IP address should be resolvable to the name provided in response to HELO/EHLO.
You should NEVER create a PTR record, whose result could not be resolved to the original IP address.
If your sending mail server is not one of your MX records (or the domain in your HELO/EHLO response does not match your mail domain example.com), you may provide an additional SPF record for example.com (see Sender Policy Framework at Wikipedia).