There are at least these two issues I have observed for this problem: 1) It could be either because your sender username or password might not be correct 2) Or it could be as answered by Avinash above, the security condition on the account. Once you try SendMail using SMTP, you normally get a notification in to your account that it may be an unauthorized attempt to access your account, if not user can follow the link to turn the settings to lessSecureApp. Once this is done and smtp SendMail is tried again, it works.