450: mailbox temporarily unavailable
The server did not take the message for this mailbox for now, because the mailbox is busy or temporarily blocked by policy. The sending server will try again.
- Standard
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 450
- Provider
- Standard SMTP codes
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
450 Requested mail action not taken: mailbox unavailable
What it means
RFC 5321 defines 450 for a mailbox that cannot take mail at the moment, and gives a busy mailbox or a temporary block for policy reasons as examples. It is the temporary partner of 550. Servers use it for greylisting, where a sender they have not seen before is asked to come back later and is accepted on a retry, and for temporary blocks while they assess a sender. The sending server keeps the message and retries on its own. If 450 replies continue until the retry period ends, the message bounces, and the enhanced code and text explain why.
Common causes
- Greylisting: the server defers the first attempt from a sender it does not know yet.
- The mailbox is locked or busy, for example during a move or a maintenance task.
- A temporary policy block, such as a rate limit or a reputation check that is not finished.
How to fix it
- 01Let the sending server retry; greylisting normally lets the message in on a later attempt.
- 02Make sure retries come from the same IP range and sender address, which is what greylisting remembers.
- 03Read the enhanced code and text for the actual reason.
- 04If it never clears, contact the recipient's administrator with the full bounce.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
Related codes
- 550Mailbox unavailable.The receiving server permanently refused the message for this recipient: the mailbox does not exist, cannot be reached, or the server rejected the message by policy.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
- 5.2.1Mailbox disabled.The recipient's mailbox exists but has been switched off and is refusing messages, for a while or for good depending on the class of the code.
- 5.7.1Delivery not authorised.The receiving server refuses the message because this sender is not allowed to send to this recipient, as a result of filtering by host or by recipient.
450 at other providers
- 450Deferred for mail to unknown addresses.GMX deferred your mail with a 450 because your server has been sending to too many GMX addresses that do not exist.
- 450Too many messages in a short time (450).GMX blocked your system for a while with a 450 because it received too many messages from it in a short period. The limit can be raised on request.
- 450Recipient is receiving too fast.GMX deferred mail to one recipient because it was being sent too often or with too many parallel attempts. It will be accepted on a later retry.
- 450Mailbox temporarily unavailable.The GMX mailbox is unavailable at the moment, so the message was deferred. Your server will retry several times; if it lasts, reach the recipient another way.
- 450E-Mail Made in Germany requirements not met.GMX rejected the connection because it did not meet the requirements of E-Mail Made in Germany (EMiG), usually because of missing or inadequate encryption.
- 450Deferred for a changing HELO or unknown addresses.IONOS delayed your mail because your server keeps changing its HELO/EHLO name, or because it sent too much to addresses that do not exist.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce