SendCommentMail sends an email to the blog owner notifiying him a new comment has been left.
You're correct that contact.aspx was setting Mail.Sender which causes some problems. SendCommentMail was not setting Sender, but setting the From address to the email address of the person leaving a comment. This too can cause problems since
some mail servers don't want to send email out when the From or Sender is not an email address managed by that email server -- and can be considered spoofing.
SendCommentMail was changed a little while back in one of the minor builds. It now sets Mail.From to the email address of the blog owner (on the Settings tab):
mail.From = new MailAddress(BlogSettings.Instance.Email);
SendCommentMail also only sends out an email if the person leaving a comment is not logged into the blog. So if you're logged in and leaving test comments, you won't get an email notification.
Also, on the Settings tab in the control panel, in the E-mail section, there's a setting labeled "Send comment e-mail". I believe that needs to be checked in order for SendCommentMail to send you an email notification.
Lastly, make sure the SendCommentMail extension is Enabled on the Extensions tab.