domain name system – Base64 encoded MX records in TXT records

You need to ask the owner of those zones as they are theoretically responsible for the content, not the DNS provider.

It may be for some sort of “authentication” as TXT records are often (ab)used for that, or just some bad tool. Or loose attempts at some sort of zone integrity checks.

Anyway, they don’t “match”. has currently 3 MX records, none of them using as mail exchanger.

In fact, instead of asking Namecheap you should ask the company behind as this is the base of the email exchangers used in the 2 domains you gave as example. Hum…. seems to be Namecheap anyway too 🙂