Enhanced SMTP status code · RFC 3463 · IANA
5.2.1 Mailbox disabled
The mailbox exists but does not accept messages. This may be permanent (a closed mailbox) or temporary.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code can be temporary or permanent.
Also seen as:
4.2.1 - Subject: X.2 · Mailbox
Something about the recipient's mailbox; under the recipient's control.
How 5.2.1 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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550 5.2.1
The email account that you tried to reach is inactive.
The recipient's account is inactive and does not accept mail.
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550 5.2.1
The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered.
The recipient is getting mail at a rate that prevents delivery, and the message was rejected. Send it again later.
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450 4.2.1
The user you are trying to contact is receiving email too quickly. Please resend your message at a later time. If the user is able to receive email at that time, your message will be delivered.
The recipient is getting too much mail too quickly, so Gmail defers delivery. The sending server retries on its own; the message arrives once the rate drops.
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450 4.2.1
The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered. Please resend your message at a later time. If the user is able to receive email at that time, your message will be delivered.
The recipient is getting too much mail too quickly, so Gmail defers delivery. The sending server retries on its own; the message arrives once the rate drops.
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450 4.2.1
Peak SMTP relay limit exceeded for this customer. This is a temporary error.
The peak limit of Google Workspace's SMTP relay service was exceeded for the sender's organisation. It is temporary; the Workspace administrator sees the limits.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026