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(); 
    }
}