SMTP Error 453

What is SMTP Error 453?

453 is not one of the reply codes defined in RFC 5321. The 4yz transient codes the standard actually enumerates (RFC 5321 §4.2.3) are 421, 450, 451, 452 and 455 — 453 is not among them. So a literal 453 is something a specific mail server emits, not a canonical code with one fixed meaning.

What you can rely on is the first digit. Per RFC 5321 §4.2.1, a 4yz reply is a transient negative completion reply: the command failed, but the condition is temporary and the same command may succeed if you retry later. Nothing is permanently wrong with your message or addresses — so do not treat 453 like a hard bounce.

Because the code itself isn't standardized, the human-readable text after 453 is what actually tells you the cause. Read it verbatim from your logs.

What causes SMTP Error 453?

In real-world mail flows, a 453 reply almost always falls into one of these buckets:

• Rate limiting / throttling. The receiving server is deliberately slowing you down because you've sent too many messages, opened too many connections, or sent too fast in a short window. This is the most common case and is signaled by 4xx codes generally (see this email throttling guide). Providers publish daily/hourly caps — e.g. Gmail/Google Workspace and Outlook.com enforce per-account send and per-message recipient limits.
• Too many recipients. Enhanced status code 4.5.3 means exactly this. Per RFC 3463, 4.5.3 is "Too many recipients … More recipients were specified for the message than could have been delivered by the protocol." In practice this enhanced code is documented almost exclusively alongside base code 452 ("452 4.5.3 Too many recipients"), not 453 — but the underlying fix is the same either way: segment the recipient list and resend.
• Relay denied. Some gateways return a 453 relay message, e.g. Symantec/Broadcom Email Security.cloud's "453 you are trying to use me [server-x.tower-xx.messagelabs.com] as a relay, but I have not been configured to let you do this" (Broadcom KB). Here the sending IP isn't authorized to relay through that host.

Note: the older guidance that 453 means "use TLS port 587 instead of SSL port 465" or "too many bounced addresses" is not supported by any RFC or provider doc for this code. Port 465 (implicit TLS) and 587 (STARTTLS) are both valid per RFC 8314 — 465 is not deprecated, and a transport/port mismatch typically surfaces as a connection or 5xx auth failure, not a transient 453.

How do I fix SMTP Error 453?

1. Read the full reply text and any 4.5.3-style enhanced code. This single step usually identifies the bucket above. Don't act on the bare number.
2. If it's throttling (most common): back off and retry. A proper MTA already retries 4xx automatically with exponential backoff. If you're sending directly, slow your rate, reduce concurrent connections, and stay under the receiver's published limits.
3. If it's 4.5.3 "too many recipients": split the recipient list into smaller batches and resend the remainder, as RFC 3463 prescribes.
4. If it's a relay message: authorize the sending IP. Confirm the public IP you connect from is registered as an allowed outbound route on the gateway (per the Broadcom KB above), or authenticate (SMTP AUTH) instead of relaying anonymously.
5. Warm up and protect sender reputation. Sudden volume spikes from a cold IP/domain are a frequent throttling trigger. Ramp volume gradually and keep SPF, DKIM and DMARC aligned so receivers trust your mail.

If you send through Courier, transient 4xx responses from downstream email providers are handled with provider-level retries and routing, so a single 453 from one provider does not need manual intervention — check the message logs to confirm the retry/delivery outcome.