A better (but annoying) reason is there are a smattering of servers which
based on broken DKIM signatures, against the rfc.
This does not have to be against the RFC.
Sections 6.1 and 6.3 of RFC 6376 say that an unverifiable DKIM
signature is treated as though the signature was not present.
John Levine, johnl(_at_)taugh(_dot_)com, Primary Perpetrator of "The Internet
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