I don't think that you can do this with a mask, since you can't build a mask that is variable and dependant on other values.  Your best bet is to likely use a simple text editor and validate the input with a regular expression.

-Matt

You would first have to describe which type of regular expression that you want to use; there is an article discussion regular expressions here.  You would then have to handle the Validating event of the UltraTextEditor (which is an event on the base class Control, so this would work on any editor) and check to see if the regular expression is a match:

private Regex emailRegEx;
private Regex EmailRegEx
{
    get
    {
        if (this.emailRegEx == null)
            this.emailRegEx = new Regex(@"\b[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b", RegexOptions.IgnoreCase);

        return this.emailRegEx;
    }
}

private void ultraTextEditor1_Validating(object sender, CancelEventArgs e)
{
 if(!this.EmailRegEx.IsMatch(this.ultraTextEditor1.Text))
     e.Cancel = true;
}

-Matt