SMTP 5.2.1: mailbox disabled or inactive

RFC 3463mailbox_inactiveaction: suppress

5.2.1 means the mailbox exists but is disabled, suspended or not accepting mail. It is permanent. Suppress it, and treat these addresses with more care than an ordinary hard bounce, because dormant mailboxes are frequently recycled into spam traps.

What it looks like in the log

550 5.2.1 The user's account has been disabled.

550 5.2.1 mailbox disabled, not accepting messages

Why it happens

• The account was suspended or closed by the provider or the organisation.
• The mailbox is dormant and has been switched off rather than deleted.

What to do

• Suppress permanently.
• Treat a rise in 5.2.1 as a list-age signal. These addresses are prime candidates for recycled spam traps.
• Apply a sunset policy so unengaged addresses stop being mailed before they become traps.

Related

5.1.1 · 4.2.2

Classified as mailbox_inactive, 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.