.NET Best Method to Send Email (System.Net.Mail has issues)
In .NET 4.0, SmtpClient is now disposable. The SMTP QUIT command is issued upon disposal such as when used in a using block.
I faced the same CPU utilization problem with the settings described. I ended up opening a ticket with Microsoft to determine the cause of the problem. The CPU utilization problem lies in the ServicePoint class. Internally in the ServicePoint class, there is a timer that runs every (MaxIdleTime/2) milliseconds. See the problem? By changing the MaxIdleTime value to 2, the CPU utilization will drop down to normal levels.
I've always used Quiksoft's EasyMail .NET components with no issues at all.
Product home page: http://www.quiksoft.com/emdotnet/
They also have a free version of the component if you only need to send out e-mails: