• A From field value longer than 2048 octets (not counting the line
  breaks of folding or the white space round it) is not read for a
  mailbox: from_mailbox gives 'none', so the Verifier's result is
  "matching From is not authentic" and the Signer does not sign.
  $Mail::DKIM2::Address::MAX_FROM_LENGTH holds the limit. Every
  implementation in the interop repository now has it; one of
  them took tens of seconds to parse a hostile 100 KB From.
  
• The Verifier waits no longer than 20 seconds in all for one
  message's DNS lookups ($Mail::DKIM2::Verifier::LOOKUP_BUDGET):
  after that the lookups not yet made fail at once, and the result
  is temperror, as for any DNS failure. A message can name 8 keys
  per signature and 32 signatures, each lookup worth the
  resolver's whole timeout. Lookups made by a PubkeyCallback or a
  PolicyCallback are not counted.