Gmail 450 4.2.1: recipient receiving mail too fast
450 4.2.1Recipient is receiving mail too fastTemporary: the sender retriesFrom Gmail and Google Workspace
450-4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered. Please resend your message at a later point. - gsmtp
What it means
Gmail limits how quickly a single mailbox can receive mail. When a recipient crosses that threshold, further messages get this 4.2.1 deferral, usually worded like the example. The issue is with the recipient mailbox, not your server reputation; any sender writing to that address at the same moment can see it.
Your server should hold the message and retry. Most deferrals clear within an hour or so. If the same mailbox keeps triggering it, something is sending it an unusual amount of mail, often a loop or an automated system.
Likely causes
- A mail loop between an auto-reply and another auto-responder or forwarding rule.
- A monitoring or alerting system sending hundreds of notifications per minute to one inbox.
- A Workspace mailbox used as a catch-all or shared inbox for heavy automated traffic.
- The recipient is being mail-bombed by a subscription attack.
How to fix it
- As a sender, let the queue retry; do not resend manually, which adds to the flood.
- If you run the system sending heavy automated mail, batch notifications into digests or spread them over time.
- As the recipient admin, look for loops in Email Log Search and break the forwarding or auto-reply cycle.
- Route high-volume machine mail to a Google Group or to a dedicated mailbox instead of a person.
Checks that apply
- Email Header Analyzer Reveals repeated Received hops or auto-reply headers that point to a loop.
- MailFlow Monitor Tracks whether test messages to the mailbox are delivered once the rate settles.
FAQ
- Is my server blocked by Gmail?
- No. This deferral is about the recipient mailbox. Mail to other Gmail users is unaffected.
- How long should the queue keep retrying?
- Standard retry schedules of several days are fine; the condition usually clears in minutes to hours.