why did php yii2 mail function not working

I'm developing yii2 application. I configured my mail function as follows to send the mail to user for email verification but i'm getting swift mailer error.

This is my configuration :

'transport' => [
               'class' => 'Swift_SmtpTransport',
               'host' => 'mail.dobuyme.com',
               'username' => 'no-reply@dobuyme.com',
               'password' => 'temp1234',
               'port' => '465', // Port 25 is a very common port too
               'encryption' => 'ssl', // 
],

But once i submit the register form i'm getting the error like this

Swift_TransportException
Expected response code 354 but got code "503", with message "503-All RCPT commands were rejected with this error:
503-Sender verify failed
503 Valid RCPT command must precede DATA

How i can fix it ?