You have enabled SMTP authentication for the IP range that the user is connecting from, but the user has not configured his client to use SMTP authentication. Either configure your email client to use SMTP authentication.
h Mail Server does not include support for the VRFY verb.
When an email client delivers an email message to an email server, it starts of by telling the server the senders address.
If a SMTP client authenticates but the username or password is incorrect, or the account is disabled, h Mail Server sends this error message to the client.
h Mail Server issues this error message if a SMTP client tries to specify a recipient address which is not valid (which has an incorrect syntax).
If there is a timeout while h Mail Server is waiting for a command from the SMTP client, this error message is sent to the client before h Mail Server disconnects the client. This error is generated by h Mail Server if a client sends a large chunk of data to h Mail Server not containing a newline character (command terminator).
A client should never do this, but incorrectly configured clients could cause this problem.
If an email message is rejected during SPF checks, this error message is issued.
This error means that the sender is trying to send an email to an address which he is not allowed to send to.
After it has done this, a transaction is started which is not finished before the entire message has been delivered to the email server.