Bounce Detection

FastLinkIt detects bounced emails through two mechanisms:

1. Send-Time Detection (Immediate)

When sending a mailing, SMTP errors are checked for bounce patterns:

• "mailbox not found", "user unknown", "does not exist"
• SMTP codes: 550 5.1.1, 550 5.1.2, 550 5.4.1
• DNS failures

Contacts are flagged as bounced immediately.

2. IMAP Bounce Checking (Background)

The BounceCheckService polls the bounce mailbox every 5 minutes for NDR (Non-Delivery Report) emails. It classifies messages using:

• RFC 3464 DSN — standard delivery status notifications
• Sender address — mailer-daemon@, postmaster@
• Subject patterns — "undelivered", "delivery failure", "returned mail"

When a bounce is detected:

• The Contact is flagged as IsBounced = true
• The MailingRecipient status is updated to "bounced"
• The mailing's FailedCount is incremented and SentCount decremented

Bounced contacts are automatically skipped in future mailings.

Related

• Mailing Statistics & Geo Report
• Subscribe Forms