SMTP 550 5.1.1 "recipient address rejected" spikes in the agent's outreach - diagnosing a dead email list

TL;DR: Stop sending to the list and validate it before the next run - a spike of 550 5.1.1 means the addresses do not exist, and continuing to send tanks the sender reputation. Verify each address (syntax plus a mailbox check), suppress the dead ones permanently, and find where the bad addresses entered the list so it does not refill with junk.

550 5.1.1 "recipient address rejected"

1. Pause the outreach run and pull the bounce records: group the 5.1.1 bounces by address and by list source. Expected: you see which addresses bounce and which source contributed most of them.
2. Check a sample of the bouncing addresses by hand: look for typos, truncated domains, and role-looking guesses. Expected: a clear pattern (bad scrape, stale export, typo cluster) emerges.
3. Run the full list through an email validation service (syntax, domain, and mailbox checks) before the next send. Expected: every address gets a verdict of valid, risky, or invalid.
4. Suppress every hard-bouncing address permanently: add them to the suppression list so no future campaign can select them. Expected: the suppression list grows by the bounce count and the list shrinks to verified addresses.
5. Fix the intake: validate addresses at collection time (scrape, form, or import) so dead addresses never enter the list again. Expected: the next list import carries a validation timestamp per address.

Use this when

• 550 5.1.1 bounces spike during an outreach run
• The list came from a scrape, an old export, or an unchecked import
• Bounce rates are climbing run over run

Not for this skill when

• The bounces are 4xx temporary failures (those are deferrals, not dead addresses - wait and retry)
• The rejection is 5.7.1 content-based (the message is the problem, not the address)
• A single address bounces (check that one address; this skill is for list-level rot)

Variant phrasings

• SMTP 550 recipient address rejected bulk
• diagnosing dead email list outreach
• 5.1.1 user unknown spikes in cold outreach

Why it happens

550 5.1.1 is the receiving server saying the mailbox does not exist. Lists rot: people change jobs, domains die, scrapes capture truncated or guessed addresses. An agent that never validates what it collected keeps mailing a growing pile of dead addresses, and receivers read the bounce rate as a spammer signal.

Edge cases

• A domain-wide 5.1.1 spike can mean the whole domain is gone (company shut down or renamed); suppress the domain, not just the addresses.
• Catch-all domains accept everything at SMTP time and bounce later; a mailbox check that only tests the domain will call them valid.
• Never re-add suppressed addresses on a list refresh; dedupe every import against the suppression list first.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_xf8iMaguzuw4naHAoTmD7g