Please follow the following steps if you are getting the following error while sending out emails from your email clients :-
"The message could not be sent because the server rejected the sender's e-mail address. The sender's e-mail address was 'XXXX@XXXX.com'. Subject 'email for smtp', Account: 'XXX.XXX.com', Server: 'XXX.XXX.com', Protocol: SMTP, Server Response: '551 This mail server requires authentication before sending mail from a locally hosted domain. Please reconfigure your mail client to authenticate before sending mail.', Port: 2525, Secure(SSL): No, Server Error: 551, Error Number: 0x800CCC78"
The above issue you are facing is due the SMTP setting done in your email client. You need to enable "SMTP authentication" on your email client. You can find the smtp authentication under the email account setting in your email client.
To enable SMTP authentication in email client please click on corresponding email client