In my current dovecot setup, incoming mails are classified by rules with growing complexity. As configuration mistakes are to be expected, storing information about filtering decisions in mail headers seems reasonable. An easy way to accomplish that would be the sieve extension editheader, which allows to specify :last to append to the end of headers.

Are there any practical reasons to chose a specific location for adding custom headers into Internet Message Format?

All reasons I considered so far are by themselves inconclusive:

  • spamassassin appends its headers
  • A consistent header order of headers is optimal for readability.
  • Forwarded mail reveals no consistent pattern as to what position large mail providers prefer.
  • Leaving signatures intact does not seem important during final delivery.
  • RFC5322 (Draft) recommends to avoid reordering without clarifying how this limits insertion.
  • The reasoning in RFC7601 (Proposed) seems applicable to filtering mechanisms as well.

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