What Is SPF in Email?
SPF, or Sender Policy Framework, is a DNS record that lists which servers are allowed to send email for your domain. A receiving mail server checks the sending server's address against that list and uses the result to decide whether the message is plausible.
What SPF checks is narrower than people assume. It validates the envelope sender, the address used during the SMTP conversation, not the From address your recipient actually sees. A message can pass SPF and still show any From address at all, which is why SPF alone was never enough.
Forms break SPF in a specific and common way. If your contact form sends mail From the visitor's own address so that replying is easy, you are sending mail claiming to be from gmail.com out of a server Google never authorised. That fails SPF, and repeated failures damage your domain's reputation.
The fix is the convention every mature form backend uses: send From an address you control, and put the visitor's address in Reply-To. Authentication passes, and hitting reply still reaches the person.