Basic SMTP reply code · RFC 5321

450 Mailbox unavailable (temporary)

The action was not taken: the mailbox is busy or temporarily blocked, for example for policy reasons.

Class: 4yz · Temporary failure
4yz: a transient failure. The command was not accepted, but may succeed if repeated; the sending server should try again (RFC 5321 §4.2.1).
Associated enhanced codes
The enhanced codes that usually come with this code, per the registry and the providers:

4.2.44.7.04.7.154.2.14.4.3174.4.244
Source
RFC 5321

How 450 appears at providers

The exact text from the provider's documentation, as you see it in the bounce, and what it means.

450 4.2.1The 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.
450 4.2.1The 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.
450 4.2.1Peak 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.

450 4.4.317Cannot connect to remote server [Message=UntrustedRoot]

The recipient's server sends its TLS certificate without the full chain, and Exchange Online cannot verify it. Configure the full chain on the server. Check with SMTP server check.
450 4.4.244The HVE message can't be submitted because the recipient rate limit was exceeded.

The HVE (High Volume Email) service throttled sending: too many messages to too few recipients in a short time.

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026