Unable to connect to mx1/mx2 comcast.net via IPv6

Hello About_discoverypatterns and welcome,

You can call 800-391-3000, use the high speed internet technical option, then ask the technical representative what is going on.The MX record address names you provided do not look like they are correct. Also, I thought all Comcast email servers were only IPv4 based but I could be mistaken.

There are AAAA records for the two MX hosts. This is MX1:

$ dig aaaa mx1.comcast.net

; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.37.rc1.el6_7.7 <<>> aaaa mx1.comcast.net
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18785
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;mx1.comcast.net.               IN      AAAA

;; ANSWER SECTION:
mx1.comcast.net.        233     IN      AAAA    2001:558:fe16:1b::15

 On another server, I can connect to the IPv6 just fine:

$ telnet 2001:558:fe16:1b::15 25
Trying 2001:558:fe16:1b::15...
Connected to 2001:558:fe16:1b::15.
Escape character is '^]'.
220 resimta-po-05v.sys.comcast.net comcast ESMTP server ready
QUIT
221 2.0.0 resimta-po-05v.sys.comcast.net comcast closing connection
Connection closed by foreign host.

 I'll give the number a call tomorrow and see what I can learn.

You can also login to your Business Class Portal (BCP) using the Administrator account, then under Manage Services, click EMAIL, then click 'View DNS Info' and this should have the exact MX record addresses that you should be using for your specific domain. These are your domain's zone file MX records that must be used to support your  BCP email accounts.

Hope this helps you out.