three.co.uk email security: SPF, DKIM & DMARC

Recommendations

1. 1Upgrade your DMARC policy from p=quarantine to p=reject for full blocking▾

   With p=quarantine, spoofed emails are sent to spam instead of being blocked outright. Some recipients still check spam folders, and sophisticated attacks can be flagged as legitimate by users. p=reject ensures fraudulent emails never reach any folder.

2. 2Harden your SPF by replacing ~all with -all (hardfail)▾

   With ~all (softfail), unauthorized senders are flagged but emails are usually still delivered. Switching to -all (hardfail) explicitly tells receiving servers to reject emails from unauthorized sources, providing much stronger protection against spoofing.

3. 3Add MTA-STS to enforce TLS encryption for incoming emails▾

   Without MTA-STS, an attacker performing a man-in-the-middle attack can downgrade the connection between mail servers to plaintext, intercepting emails in transit. MTA-STS tells sending servers to only deliver via TLS with a valid certificate, preventing downgrade attacks.