As SMTP protocol specifies, when email message is posted
to Smtp server, client must specify sender of the message.
For that reason he must issue "MAIL
FROM:" command. When this happens,
wodSmtpServer will fire MailFrom
event informing application about sender. You can specify
if you will allow to receive email messages from
such user, or just deny action.
After event is finished, and during client's
posting, this property will hold email address that client
specified.