The SendUsing is throwing an error because (at least in the code snippet you gave us) you have not defined an SMTP server.
myMail.SMTPServer = "mail.mycompany.com"
By default, if SmtpMail.SmtpServer is not set, System.Web.Mail is supposed to use localhost as the default property. However, for some reason this doesn't appear work. If you are using the local SMTP Service to send emails, you may try the following values:
That should fix it.