450 4.2.1 Gmail bounce: receiving mail too fast
What 450 4.2.1 means
The recipient mailbox is receiving mail too quickly, so Gmail defers delivery to it for now.
Message text
The user you are trying to contact is receiving email too quickly…
This is an excerpt: the provider's documentation shortens the message. Full text: provider documentation
Common causes of 450 4.2.1
- Recipient receives mail faster than its rate cap allows, often during a campaign or mail storm
How to fix 450 4.2.1
- Slow the send rate to that address and retry later.
- Check the recipient is not on a list that receives several messages at once.
Check your domain
Read the headers of a bounced messageRuns in your browser; nothing is uploaded.
Get your domain's deliverability score
Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 450 4.2.1
The first part, 450, is the SMTP reply code. Requested mail action not taken, mailbox unavailable: the mailbox is busy or temporarily blocked. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 4.2.1, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 4 is a Persistent transient failure (4xx): the command was not accepted, but the condition may clear, so the sender should try again later (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 2 is mailbox: a problem with the recipient mailbox, such as full, disabled or unknown. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Sending servers should retry a deferred message. RFC 5321 §4.5.4.1 advises waiting at least 30 minutes between attempts and trying for at least 4 to 5 days before giving up and returning a bounce. If the deferral keeps repeating, fix the cause rather than waiting.
Official source: knowledge.workspace.google.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 4.2.2 SMTP: mailbox full
- 452 4.2.2 Gmail: recipient inbox full
- 421 4.7.0 Gmail: no PTR record
- 451 4.7.23 Gmail: PTR record missing
- 421 4.7.28 Gmail: rate limited
- 451 4.7.24 Gmail: spf record suspicious
FAQ
What does 450 4.2.1 mean?
The recipient mailbox is receiving mail too quickly, so Gmail defers delivery to it for now. This is a temporary failure.
How do I fix 450 4.2.1?
Slow the send rate to that address and retry later. Check the recipient is not on a list that receives several messages at once.
Is 450 4.2.1 permanent, and should I retry?
450 4.2.1 is a temporary failure. Your mail server normally retries by itself; if it keeps repeating, fix the cause.