How does SPF validation actually happen during SMTP?
During SMTP, the receiving server performs the SPF check before accepting the content. The steps are:
The sending server connects.
The receiving server takes the envelope from domain.
It performs a DNS TXT lookup.
It evaluates mechanisms such as ip4, include, and mx.
It applies the qualifier of the first matching rule.
This all occurs behind the scenes before the final acceptance of the message.
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