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Email error 450 4.2.1: Mailbox temporarily unavailable

The mailbox exists but cannot take mail right now. It may be busy, locked or limited. Most servers retry on their own.

Temporary failure (soft bounce)

What it means

The mailbox exists but cannot take mail right now. It may be busy, locked or limited. Most servers retry on their own.

Common causes

  • The receiving server is overloaded.
  • The mailbox is being moved or maintained.
  • The recipient has received too much mail in a short time.
  • A temporary policy limit is active.

How to fix it

  1. Wait and let the sending system retry.
  2. If it repeats for days, treat the address as risky.
  3. Space out sends to the same domain.
  4. Verify addresses first so that real problems stand out from valid ones.

How to prevent it

Most bounces come from addresses that were never checked. Verify each address before you send: syntax, domain, mailbox and risk are checked without sending a message. Test a single address with the free email checker or upload a whole list for bulk verification.

Verify an email address free · Clean a whole list · MX lookup · SPF, DKIM and DMARC checker

Frequently asked questions

Is the address valid?

Usually yes. A temporary code means the mailbox probably exists.

Should I delete it?

No, not on the first occurrence.

Other error codes

550 5.1.1User unknown (mailbox does not exist) 550 5.7.1Message rejected by policy (blocked) 550 5.7.26Sender not authenticated (SPF/DKIM/DMARC) 550 5.4.1Recipient address rejected: access denied 552 5.2.2Mailbox full (over quota) 553 5.1.3Invalid address syntax

All email error codes