What is the difference between 421 and 451?

421 usually means the service is completely unavailable and closing the connection, while 451 means the server is running but encountered an error processing the specific command or message.

Do I need to change my email content?

Usually no, as this is a server-side issue. However, if a spam filter crashed while scanning your email, simplifying the content might help.

← Error library

451 4.3.0 Temporary mail server error

temporary Deferral

Meaning

The receiving email server encountered a temporary internal error and cannot process the message at this moment.

Common causes

The recipient's mail server is experiencing an internal processing failure.
An anti-spam or anti-virus filter on the receiving end crashed or timed out.
The server is undergoing unexpected maintenance.

How to fix

Your mail server will automatically queue the message and retry later.
Monitor the delivery queue; no immediate action is required on your part.
If the issue persists for days, contact the recipient via another channel to alert their IT team.

Provider notes

Gmail. Can mean 'Email server has temporarily rejected this message' or 'Multiple destination domains per transaction is unsupported.'

Example bounce

451 4.3.0 Email server has temporarily rejected this message.

FAQ

What is the difference between 421 and 451?: 421 usually means the service is completely unavailable and closing the connection, while 451 means the server is running but encountered an error processing the specific command or message.
Do I need to change my email content?: Usually no, as this is a server-side issue. However, if a spam filter crashed while scanning your email, simplifying the content might help.