The SMTP 501 error occurs when you try to send an email to an invalid email address or invalid domain name. Additionally, you may run into this error from time to time due to anti-virus guards dropping your SMTP connection or if the SMTP command exceeds 512 characters (when sending the email from the command line).
Furthermore, specific SMTP servers throw the 501 error when the email address does not comply with the RFC 2821 Specifications.
For command-line users
If you're sending the email using an SMTP command exceeding 512 characters, you can use the SMTP extensions to increase this limit.
For non-command-line users
First, verify the validity of the sender's email address and the sender's domain. If the email is invalid, use the valid email address when you send the email to resolve the error.
However, if the email you are trying to send is valid, check if your SMTP Server complies with the RFC 2821 specifications. If so, make sure the sender's email is in the following format:
If your email address complies with these standards and the error persists, a quick fix would be to disable your firewall or anti-virus guard.
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