501
SMTPERRORCommonClient ErrorHIGH confidence
Syntax error in parameters or arguments
What this means
The 501 code indicates the command was recognized, but its parameters or arguments were invalid. This could be a malformed email address or an incorrect value for a command option.
Why it happens
- 1The email address in `MAIL FROM` or `RCPT TO` is not properly formatted (e.g., missing '@' or domain).
- 2Invalid characters are present in the arguments.
- 3The parameters do not conform to the expected syntax for the given command.
How to reproduce
A client provides an invalid email address in the `MAIL FROM` command.
trigger — this will error
trigger — this will error
MAIL FROM:<sender-example.com> 501 5.1.7 Bad sender's mailbox address syntax
expected output
501 Syntax error in parameters
Fix
Validate email address syntax
WHEN Before sending the `MAIL FROM` or `RCPT TO` command
Validate email address syntax
// Use a regex or library to validate email address format
const email = "bad-email";
if (!isValidEmail(email)) {
throw new Error("Invalid email format");
}Why this works
Ensures that malformed addresses are not sent to the mail server, preventing this error.
What not to do
✕ Retry without fixing the parameters
The server has permanently rejected the parameters as invalid. It will not work on a retry.
Sources
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