Temporary failure (soft bounce)
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.
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
- Wait and let the sending system retry.
- If it repeats for days, treat the address as risky.
- Space out sends to the same domain.
- Verify addresses first so that real problems stand out from valid ones.
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
Let's Bite! Get Free Credits
Get 100 Free Credits Monthly
Try first, decide later! Get started with a free plan.
Verify 100 email addresses free every month. Plans start at $19.90 a month for 25,000 verifications.