SMTP Error 221 – Domain service closing transmission channel

SMTP Error 221 – Domain service closing transmission channel
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What does the Error means?

SMTP Error 221 response is sent from the server when the remote server/client has notified that it is closing the connection. You would commonly notice this command after a QUIT command was received.
This also states that the the session or connection to the mail server is now ending. Suggesting that all processes have completed, this could be successful or due to failures.

SMTP Status 221 is often misconstrued as an error condition, when it is in fact nothing of the sort. The mail server is simply telling you that it has processed everything it was given in this particular session, and it is now going back into waiting mode.

Error 221 is an authentication error, which means you are using the wrong mail server, the user/password is incorrect, you need to change the authentication method or SMTP access is not enabled for your email account. Try using SSL/TLS if you are sure that the mail server and user/password is correct.Error Description: This response is sent from the server when the remote server/client has notified that it is closing the connection. You would commonly notice this command after a QUIT command was received.

Error Message

221 Domain service closing transmission channel
221 Closing transmission
221 Goodbye