SMTP 5.1.1: recipient address does not exist

RFC 3463invalid_recipientaction: suppress

5.1.1 means the mailbox does not exist at the receiving domain. It is permanent. Suppress the address immediately. Continuing to send to addresses that return 5.1.1 is one of the fastest ways to damage sender reputation.

What it looks like in the log

550 5.1.1 <user@example.com>: Recipient address rejected: User unknown in virtual mailbox table

550 5.1.1 The email account that you tried to reach does not exist.

550 5.1.1 unknown or illegal alias

Why it happens

• The address was mistyped at collection.
• The mailbox was deleted, commonly after someone leaves an organisation.
• The address was never real, which points at list quality or a lack of confirmed opt-in.

What to do

• Suppress permanently on the first occurrence. There is no reason to retry.
• If the hard bounce rate is above about 2%, the problem is list acquisition, not delivery.
• Validate at the point of capture rather than discovering invalid addresses at send time.

Related

5.2.1 · 4.2.2

Classified as invalid_recipient, action suppress. Paste a response into the bounce classifier to check one against the same ruleset.

Written and maintained by Rastu Singh, an infrastructure engineer working on high-volume email in Tallinn.