The command parameters contain syntax errors, often due to malformed email addresses.
SMTP error 501 means the command was recognized but its parameters were invalid:
The email address contains invalid characters or is improperly formatted.
Unencoded special characters in the sender or recipient fields.
Character encoding problems in email headers or addresses.
Check that all email addresses are properly formatted (user@domain.com) without extra spaces or invalid characters.
Remove or properly encode special characters in display names and addresses.
Ensure your mail client uses standard UTF-8 encoding for international characters.
501 5.1.3 Invalid address format
501 Syntax error in parameters or arguments
501 5.5.4 Invalid address