SMTP E-Mail Sender in C#

Either a call to Dispose on your SmtpClient after you're done using it:

smtp.Dispose();

or use a using:

using (SmtpClient smtp = new SmtpClient
{
Host = "smtp.gmail.com",
Port = 587,
Credentials = new NetworkCredential("user@gmail.com", "password"),
EnableSsl = true
})
{
smtp.Send(mail);
}

The using will take care of calling Dispose for you.

Also, don't forget that there's a convenience method on the SmtpClient, if you don't need anything fancy on the message like HTML formatting or attachments.

smtp.Send(from, to, subject, body);