Gmail 450 4.2.1
Recipient receiving mail too fast
Gmail 450 4.2.1 means: recipient receiving mail too fast. It is a temporary failure, so your server retries it. The recipient, or your relay account, is over a Gmail rate limit. Retry later and spread sends to the same recipient over time.
Gmail’s exact wording
Quoted from Google Workspace Admin Help: Gmail SMTP errors and codes, checked 2026-11-05. Placeholders are Gmail’s own.
Reply text
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. For more information, go to Limits for sending & getting email.
Reply text
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. For more information, go to Limits for sending & getting email.
Reply text
Peak SMTP relay limit exceeded for this customer. This is a temporary error. For more information on SMTP relay limits, contact your administrator or go to SMTP relay service error messages.
| Who sends it | Gmail and Google Workspace |
|---|---|
| Basic code | 450: Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons) (RFC) |
| Enhanced code | 4.2.1: Mailbox disabled, not accepting messages |
| Permanent or temporary | Temporary (4xx) |
| Verifying the list prevents it | No |
In practice
This is a per-recipient rate limit. Gmail throttles a single user who is receiving mail very quickly, which often happens with automated alerts or a loop sending to the same address. The third version, peak SMTP relay limit exceeded, applies to Google Workspace customers relaying through smtp-relay.gmail.com. In both cases the message is retried and usually delivered once the rate falls.
What the sender should do
The recipient, or your relay account, is over a Gmail rate limit. Retry later and spread sends to the same recipient over time.
Would verifying the list have prevented Gmail 450 4.2.1?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Gmail 450 4.2.1 mean?
- Gmail 450 4.2.1 means: recipient receiving mail too fast. It is a temporary failure, so your server retries it. The recipient, or your relay account, is over a Gmail rate limit. Retry later and spread sends to the same recipient over time.
- Should I retry after Gmail 450 4.2.1?
- Your sending server retries it automatically, because it is a temporary (4xx) failure. Investigate only if it keeps coming back over several hours.
- Would verifying my list have prevented Gmail 450 4.2.1?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
The same code elsewhere
Sources
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 5321: Simple Mail Transfer Protocol, section 4.2.3 (reply codes) (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05