An API I dont't known. For a simple solution to start the email-client with an address you can use:
System.Diagnostics.Process.Start("mailto:"+emailAddress);
But its only for simple single emails and the limitations are obvious. Make sure to catch the exceptions that could be thrown.
Update
I Have not seen the attachment requirement. Can not help in this case but anyhow here sample-code if you have outlook and the interops installed. Maybe it helps you:
Microsoft.Office.Interop.Outlook.Application app =
new Microsoft.Office.Interop.Outlook.Application();
Microsoft.Office.Interop.Outlook.MailItem mailItem=
app.CreateItem(Microsoft.Office.Interop.Outlook.OlItemType.olMailItem);
mailItem.Attachments.Add(filePath);
// ....