Well done for first steps. Even (one of) mail servers' IPv6 record has a valid reverse record, very nice!

Now, how about adding reverse to the main AAAA too?

Also, what's up with one of MX'es lacking IPv6 address? With both mail.linkedin.com and mail-c.linkedin.com having the same priority (10), that's a recipe for 50% of delivery attempts randomly failing to IPv4.

  $ dig +short aaaa linkedin.com
  2620:109:c002::6cae:a0a

  $ dig +short -x 2620:109:c002::6cae:a0a
  (no reply)

  $ dig +short mx linkedin.com
  10 mail-c.linkedin.com.
  10 mail.linkedin.com.

  $ for i in mail.linkedin.com. mail-c.linkedin.com.; \
    do echo $i; dig +short aaaa $i; done
  mail.linkedin.com.
  (no reply)
  mail-c.linkedin.com.
  2620:109:c006:104::215

  $ dig +short -x 2620:109:c006:104::215
  mail-c.linkedin.com.