How do blocklists trigger SMTP rejections?

Blocklists integrate into SMTP rejection flow:

How it works:

Sender connects to receiving server. Receiver queries blocklist (DNS lookup). If IP or domain is listed, connection rejected. Rejection message often references the blocklist.

Query timing:

Often at connection time (before MAIL FROM). Sometimes after RCPT TO. Rarely at DATA phase.

Typical rejection flow:

SMTP 550 or 554 response. Message includes blocklist name. May include lookup URL. Example: "550 Blocked by Spamhaus SBL"

Blocklist types queried:

IP blocklists: Spamhaus SBL, Barracuda, Sorbs. Domain blocklists: Spamhaus DBL, SURBL. URL blocklists: Checking links in content.

Resolution:

Identify which blocklist. Follow their delisting process. Address underlying cause.

Blocklist rejection is the port authority checking your ship's record. If flagged, you are turned away.