SMTP 504 Parameter Not Implemented vs 521 Host Does Not Accept Mail
Both SMTP 504 (Parameter Not Implemented) and 521 (Host Does Not Accept Mail) belong to the 5xx Permanent Negative category. 504 indicates that a command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported. Meanwhile, 521 means that the host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
Description
A command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported.
When You See It
When using SMTP extensions or parameters that the server does not support, such as an unsupported AUTH mechanism or an unrecognized MAIL FROM parameter.
How to Fix
Check the server's EHLO response for supported extensions and parameters. Use only the mechanisms and parameters listed in the server's capability advertisement.
Description
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
When You See It
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
How to Fix
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.
Key Differences
SMTP 504: A command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported.
SMTP 521: The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
You encounter 504 when when using SMTP extensions or parameters that the server does not support, such as an unsupported AUTH mechanism or an unrecognized MAIL FROM parameter.
You encounter 521 when when connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
When to Use Which
For 504 (Parameter Not Implemented): Check the server's EHLO response for supported extensions and parameters. Use only the mechanisms and parameters listed in the server's capability advertisement. For 521 (Host Does Not Accept Mail): Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.