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.