450 4.2.1: peak SMTP relay limit exceeded
Your Google Workspace organization is sending through smtp-relay.gmail.com faster than its peak relay limit allows, so Google is deferring relay mail for now.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 450 4.2.1
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
450 4.2.1 Peak SMTP relay limit exceeded for this customer. This is a temporary error.
What it means
This comes from the Google Workspace SMTP relay service, the smtp-relay.gmail.com route that lets an organization's own servers, applications and devices send mail through Google. Google sets a peak limit on how much relay mail one customer can send in a short period. When your organization goes over it, relay mail is deferred with this temporary error instead of being rejected, and messages that stay queued on your sending server go out once the rate drops. Google's relay error page points admins to the SMTP relay service setting for the limits that apply to their account.
Common causes
- An application, printer or server is sending a large batch through the relay in a short burst.
- Several systems in the organization relay through Google at the same time and their traffic adds up.
- A misconfigured device or script is resending the same messages in a loop.
How to fix it
- 01Let the sending server retry: this is a 4xx deferral, and queued mail goes out once the rate drops.
- 02Spread large sends over a longer period, or throttle the application that relays the most mail.
- 03Check the SMTP relay service setting in the Admin console and Google's relay documentation for your account's limits.
- 04Look for a device or script that is resending the same messages, and stop it.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
- SMTP relay service error messages (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 550 5.7.1Daily relay limit reached for the customer.Your Google Workspace organization has relayed as much mail through smtp-relay.gmail.com today as its daily limit allows.
- 550 5.4.5Daily relay limit reached for a user.A Workspace user has sent more mail through smtp-relay.gmail.com today than the SMTP relay service allows for one user.
- 450Mailbox temporarily unavailable.The server did not take the message for this mailbox for now, because the mailbox is busy or temporarily blocked by policy. The sending server will try again.
Further reading
All 108 Gmail codesEvery code in the wikiPaste your own bounce