DNS: Why so many *._domainkey.example.com prefixes?

AJ ONeal (Home) coolaj86 at gmail.com
Mon Nov 23 14:28:06 MST 2015

I've tried googling a little, but I don't know enough to figure out what my
query should be.

I notice that every time I set up mailgun or mandrill or whatever I always
have to set up domainkeys. That part makes sense - RSA, public/private, etc
- I get that.

Intuitively I would think there is a single standard for where to find SMTP
domain keys, say

But instead it seems like one of these prefixes get picked at random:
pic._domainkey.example.com (this one baffles me)

So I'm looking at my long list of domains and it appears that only those 5
variations occur, but I'm not seeing a pattern.

Why isn't there just one standard prefix?
How does that querying server know which to query?
Isn't _ an illegal character?

I'd love to be pointed to some reading material as well as some short and
sweet explanations if you have one on hand.


AJ ONeal

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