What 450 4.2.1 means
Gmail returns 450 4.2.1 when the mailbox you're sending to is receiving messages faster than Gmail allows for one user. 4.2.1 is the 'mailbox disabled / not accepting messages' status in its transient form: the mailbox exists but won't take more right now.
This limit is on the recipient, not on you. It typically hits shared inboxes (sales@, info@, support@) and role accounts that take a flood of mail, or an address being mail-bombed. Gmail's text says your message will be delivered if the user can receive mail when you resend.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
450 4.2.1 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.450 4.2.1 Peak SMTP relay limit exceeded for this customer. This is a temporary error.SMTP relay only: your Workspace organization hit its relay rate limit.
Why cold emailers see 450 4.2.1
- Sending to a busy role or shared address that receives a high volume from many senders.
- Several of your own inboxes emailing the same address within minutes, for example a mis-configured sequence or duplicate leads.
- Google Workspace SMTP relay: a separate 450 4.2.1 variant means your organization exceeded its peak relay rate.
How to fix 450 4.2.1
- Let your sending server retry later. Most 4.2.1 deferrals clear within hours.
- Deduplicate your lead list so the same address isn't emailed by multiple inboxes or campaigns at once.
- Avoid role addresses for cold outreach. They're busy, rarely the decision-maker, and more likely to report you.
- If you use Google's SMTP relay and see the 'Peak SMTP relay limit exceeded' text, spread sends over time; that one is about your own account.
Will it retry on its own?
Retry, don't suppress. If the same address returns 4.2.1 for several days, deprioritize it; a mailbox that's always flooded is a poor cold-email target anyway.
Reading the code
The basic code 450 means "Mailbox unavailable, action not taken (temporary)" under RFC 5321. In 4.2.1, the 4 is the class (persistent transient failure) and the 2 is the subject (mailbox). The detail number 1 is registered in RFC 3463 / the IANA registry as "Mailbox disabled, not accepting messages".
- Class 4
- Persistent transient failure
- A temporary condition delayed delivery. The message itself is valid, and sending again later may succeed.
- Subject 2
- Mailbox
- A problem with the recipient's mailbox, such as full or disabled. Under the recipient's control.
- Detail 4.2.1
- Mailbox disabled, not accepting messages
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.