C# how to correctly dispose of an SmtpClient?

public void SendMessage()
{
    using (SmtpClient client = new SmtpClient())
    {
        client.Send(Message);
    }
    DisposeAttachments(); 
}

That way the client will be disposed even if an exception is thrown during the Send method call. You should very rarely need to call Dispose explicitly - it should almost always be in a using statement.

However, it's not clear how the attachments are involved here. Does your class implement IDisposable itself? If so, that's probably the place to dispose of the attachments which are presumably member variables. If you need to make absolutely sure they get disposed right here, you probably need:

public void SendMessage()
{
    try
    {
        using (SmtpClient client = new SmtpClient())
        {
            client.Send(Message);
        }
    }
    finally
    {
        DisposeAttachments(); 
    }
}

The SmtpClient class in .NET 4.0 now implements IDisposable, while the SmtpClient class in .NET 2.0 lacks this interface (as Darin noted). This is a breaking change in the framework and you should take appropriate actions when migrating to .NET 4.0. However, it is possible to mitigate this in your code before migrating to .NET 4.0. Here is an example of such:

var client = new SmtpClient();

// Do not remove this using. In .NET 4.0 SmtpClient implements IDisposable.
using (client as IDisposable)
{
    client.Send(message);
} 

This code will compile and run correctly both under .NET 2.0 (+3.0 and 3.5) and under .NET 4.0.

using (SmtpClient client = new SmtpClient())
{
    client.Send(Message);
    DisposeAttachments(); 
}

Interesting - contrary to .NET 3.5, SmtpClient implements IDisposable in .NET 4.0, learning new things every day.