How to configure Reverse DNS, PTR and MX records for private email server behind Comcast Static IP

A domain registry like register.com is only authoratative for forward records like A records and MX records.  You can only setup PTR records with the hoster of the IP address block - in this case Comcast.

The by the book process is contact Comcast on the phone Mon-Fri 8-5pm Eastern time.

Incidentally your PTR isn't correct.   Take a look at the following:

Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.

C:\Users\tedm>nslookup
Default Server:  UnKnown
Address:  172.16.1.13

> set type=mx
> estarmail.com
Server:  UnKnown
Address:  172.16.1.13

Non-authoritative answer:
estarmail.com   MX preference = 0, mail exchanger = mail.estarmail.com

estarmail.com   nameserver = dns142.a.register.com
estarmail.com   nameserver = dns167.b.register.com
estarmail.com   nameserver = dns223.c.register.com
estarmail.com   nameserver = dns249.d.register.com
mail.estarmail.com      internet address = 70.91.104.193
dns142.a.register.com   internet address = 216.21.231.142
dns167.b.register.com   internet address = 216.21.232.167
dns223.c.register.com   internet address = 216.21.235.223
dns249.d.register.com   internet address = 216.21.236.249
> set type=a

> mail.estarmail.com
Server:  UnKnown
Address:  172.16.1.13

Non-authoritative answer:
Name:    mail.estarmail.com
Address:  70.91.104.193

> set type=ptr
> 70.91.104.193
Server:  UnKnown
Address:  172.16.1.13

Non-authoritative answer:
193.104.91.70.in-addr.arpa      name = estarmail.com

------>>>>>>>>>>>>>>>>>>>>>>>>>>^^^^^^^^^^^^^

(should be mail.estarmail.com)

91.70.in-addr.arpa      nameserver = dns104.comcast.net
91.70.in-addr.arpa      nameserver = dns102.comcast.net
91.70.in-addr.arpa      nameserver = dns103.comcast.net
91.70.in-addr.arpa      nameserver = dns105.comcast.net
91.70.in-addr.arpa      nameserver = dns101.comcast.net
dns101.comcast.net      internet address = 69.252.250.103
dns101.comcast.net      AAAA IPv6 address = 2001:558:fe23:8:69:252:250:103
dns102.comcast.net      internet address = 68.87.85.132
dns102.comcast.net      AAAA IPv6 address = 2001:558:1004:7:68:87:85:132
dns103.comcast.net      internet address = 68.87.76.228
dns103.comcast.net      AAAA IPv6 address = 2001:558:1014:c:68:87:76:228
dns104.comcast.net      internet address = 68.87.68.244
dns104.comcast.net      AAAA IPv6 address = 2001:558:100a:5:68:87:68:244
dns105.comcast.net      internet address = 68.87.72.244
dns105.comcast.net      AAAA IPv6 address = 2001:558:100e:5:68:87:72:244
>

Forward and reverse records only exist for hostnames not domains.

A domain (estarmail.com) can have a MX record and it can have an A record but the MX

record is only used for routing mail.  The A record is not.   CNAME records are not used

for routing mail at all.   Setting a PTR  (reverse DNS) record to point to a domain instead of

a hostname is nonsensical.

Thank you, I have updated the PTR record at REGISTER.COM and opened a ticket at Comcast.