Should I retry a 421 error?

Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.

How do I fix SMTP 421?

Let your server retry. Most 421 deferrals clear on their own within minutes or hours.

Temporary — will retry

SMTP 421

service not available, closing channel

SMTP 421 means the receiving server is not available right now and is closing the connection. RFC 5321 defines it as "service not available, closing transmission channel". It is a temporary failure: your server keeps the message queued and retries. It says nothing about whether the recipient address exists.

How it appears

421 Service not available, closing transmission channel

421 4.3.2 Service shutting down

421 4.7.0 Try again later, closing connection.

What causes it

The receiving server is overloaded, restarting or shutting down for maintenance.
You opened too many connections or sent too fast, and the server is rate limiting you.
The receiving server distrusts your IP for now, often after a spike in volume or complaints.
A network timeout ended the session before the transaction finished.

How to fix it

In order — try the first one first.

Let your server retry. Most 421 deferrals clear on their own within minutes or hours.
If one provider defers you persistently, lower the number of parallel connections and messages per connection to that provider.
Read the enhanced code and text. 4.7.x points at reputation or policy, 4.4.x at connection problems, 4.3.x at the receiver's own resources.
If the deferrals start after a big send, check bounce and complaint rates before the next one.

Common questions

What does SMTP 421 mean?: SMTP 421 means the receiving server is not available right now and is closing the connection. RFC 5321 defines it as "service not available, closing transmission channel". It is a temporary failure: your server keeps the message queued and retries. It says nothing about whether the recipient address exists.
Should I retry a 421 error?: Yes, and your sending server does it automatically. 421 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
How do I fix SMTP 421?: Let your server retry. Most 421 deferrals clear on their own within minutes or hours.

Specific 421 replies

Last reviewed 2026-09-11