On 21 May 2017, at 11:33, ComKal Networks wrote:
Anyone forwards an email I've sent them, then the headers will specify their sending domain so the SPF record for my domain should be irrelevant.
1. SPF does not operate on any email headers. It operates on the SMTP envelope sender. RFC5321.MailFrom in RFC5598-ese.
2. Most traditional forwarding mechanisms (e.g. .forward files, sendmail & postfix aliases, etc.) DO NOT modify RFC5321.MailFrom. This causes SPF "-all" breakage when recipients forward mail. Such forwarding has been common among colleges & universities for alumni and for professional organizations such as the ACM.
3. There is a "Sender Rewritng Scheme" (SRS) specified in an ID (but never in a RFC) which can be used by forwarding MTAs to avoid SPF breakage, however it is relatively uncommon among systems that don't specialize in forwarding.
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