The SMTP (Simple Mail Transfer Protocol) is the mail server that sends outgoing mail.

Navigate to Host > Host Settings.
At Advanced Settings, click the Maximize  button.
At SMTP Server Settings, click the Maximize  button.
In the SMTP Server field, enter the SMTP address of the server. E.g. mail.yourdomain.com
At SMTP Authentication, select from the following options:
Anonymous  (This is the default option)
Basic (You should choose "Basic")
NTLM (Do not choose)
In the SMTP Username field, enter a SMTP Username. (admin@yourdomain.com)
In the SMTP Password field, enter a SMTP Password. (yourpassword)
Click Update.