What happens during SMTP connection filtering?

During SMTP connection filtering, the receiving server evaluates the sender before accepting any message content. This is the first line of defense, and rejections here are efficient because no bandwidth is wasted on spam.

The server checks the connecting IP against blocklists like Spamhaus and Barracuda. It performs a reverse DNS lookup to verify the IP has a valid hostname. It checks SPF records to confirm the IP is authorized to send for the claimed domain.

Rate limiting may also apply. If the sending server is attempting too many connections or sending too quickly, the receiver may issue temporary 4xx deferrals or permanent 5xx rejections.

Connection-level filtering stops the most obvious spam. It also protects server resources by refusing to process content from known bad actors.

This is the port authority checking your ship's registration before you are allowed to approach the dock.