using (SynchronizationContextSwitcher _ctxSwitcher = new SynchronizationContextSwitcher())
{

	SmtpClient smtp1 = new SmtpClient() { Host = "" };

	using (MailMessage msg = new MailMessage() {
			From = new MailAddress(""),
			Subject = "",
			IsBodyHtml = true})
	{
		smtp.SendCompleted += (s, e) =>
		{
			if (!e.Error.Equals(null))
			{
				//...
			}

		};

		smtp.SendAsync(msg, "");
	}
}