smtp bounce code
450 4.2.1Mailbox temporarily unavailable
what to do
Retry, do not suppress
Temporary. The mailbox exists but cannot take mail this moment.
What the server is telling you
Often throttling on a busy account, or a mailbox being moved or migrated. It resolves on its own.
Next steps
- →Retry over the next day or two.
- →Do not count this as a bounce in your rate.
Commonly returned by: Exchange, hosted providers
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