v= Version of DKIM

a= Hash algorithm

c= Canonicalization

d= Domain that signed the message

s= Selector, selector helps to find the correct public key from the d value

h= Headers, specifies which headers are hashed using the hash algorithm

i= Identity of the user or agent on behalf of which the message is signed

b= Signature data

p= Public key

k= Key type

g= Glanularity

n= Notes 

bh= Hash of the canonicalised body

l= Body length count

q= Query method used to retrieve the public key

t= Timestamp of signature

x= Expiration of signature

t=y Means that DKIM is in testing mode, should be treated the same as if there was no DKIM
​
​
Mandatory tags:
If any of those are missing DKIM will return "PermError".