What are best practices for using SmtpClient, SendAsync and Dispose under .NET 4.0

The original question was asked for .NET 4, but if it helps as of .NET 4.5 SmtpClient implements async awaitable method SendMailAsync.

As a result, to send email asynchronously is as the following:

public async Task SendEmail(string toEmailAddress, string emailSubject, string emailMessage)
{
    using (var message = new MailMessage())
    {
        message.To.Add(toEmailAddress);

        message.Subject = emailSubject;
        message.Body = emailMessage;

        using (var smtpClient = new SmtpClient())
        {
            await smtpClient.SendMailAsync(message);
        }
    }
}

It's better to avoid using SendAsync method.

Note: .NET 4.5 SmtpClient implements async awaitable method SendMailAsync. For lower versions, use SendAsync as described below.

---

You should always dispose of IDisposable instances at the earliest possibility. In the case of async calls, this is on the callback after the message is sent.

var message = new MailMessage("from", "to", "subject", "body"))
var client = new SmtpClient("host");
client.SendCompleted += (s, e) => {
                           client.Dispose();
                           message.Dispose();
                        };
client.SendAsync(message, null);

It's a bit annoying the SendAsync doesn't accept a callback.

In general, IDisposable objects should be disposed as soon as possible; implementing IDisposable on an object is intended to communicate the fact that the class in question holds expensive resources that should be deterministically released. However, if creating those resources is expensive and you need to construct a lot of these objects, it may be better (performance wise) to keep one instance in memory and reuse it. There's only one way to know if that makes any difference: profile it!

Re: disposing and Async: you can't use using obviously. Instead you typically dispose the object in the SendCompleted event:

var smtpClient = new SmtpClient();
smtpClient.SendCompleted += (s, e) => smtpClient.Dispose();
smtpClient.SendAsync(...);