450 4.2.1: Recipient is receiving mail too quickly
Gmail returns 450 4.2.1 when the recipient mailbox is receiving mail faster than Gmail allows, so it defers your message. This is about the recipient’s inbound rate, not your reputation. Your server should retry later. If it recurs for one address, slow down how often you send to that address.
The exact messages
Gmail / Google Workspace
450 4.2.1 The user you are trying to contact is receiving email too quickly.
Gmail / Google Workspace
450 4.2.1 The user you are trying to contact is receiving email at a rate that prevents additional messages from being delivered.
Why it happens
- Many messages to the same recipient in a short time, from you or from all senders combined.
- An automated system or mail loop flooding the mailbox.
How to fix it
- Let your server retry on its normal schedule.
- Space out messages to the same recipient.
If you send cold email
Sequences that fire several follow-ups within minutes to the same person can hit this. Keep follow-ups days apart.
How WarmHawk handles it
WarmHawk never sends more than a mailbox’s own daily cap, spaces sends from one mailbox at least 8 minutes apart with random jitter, and starts a newly warmed mailbox at 5 campaign emails a day, growing 20% a day. A temporary failure is retried with backoff (30 minutes, then doubling) and the lead is suppressed after 4 attempts, so a throttled mailbox is never hammered. How WarmHawk works →
Related bounce codes
Sources, checked 2026-09-29: Google Workspace: Gmail SMTP errors and codes. Have a different bounce? Paste it into the decoder →
Questions
4.2.1 questions
What does 4.2.1 mean?+
Gmail returns 450 4.2.1 when the recipient mailbox is receiving mail faster than Gmail allows, so it defers your message. This is about the recipient’s inbound rate, not your reputation. Your server should retry later. If it recurs for one address, slow down how often you send to that address.
Is 4.2.1 a temporary or permanent error?+
Temporary. The sending server keeps the message and retries it for a while. It only becomes a bounce if every retry fails before the message expires.
How do I fix 4.2.1?+
Let your server retry on its normal schedule. Space out messages to the same recipient.