ShellExecuteA是shell32.dll（该DLL在系统目录里）里封装的一个函数，可以实现你的功能。
以下的代码可以用自动打开默认的邮件发送程序：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net","","",5)
以下的代码可以用自动打开默认的邮件发送程序（有主题）：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net?Subject=This is the subject","","",5)
以下的代码可以用自动打开默认的邮件发送程序（有主题，抄送）：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net?Subject=This is the subject&cc=hello@5dmedia.com","","",5)
以下的代码可以用自动打开默认的邮件发送程序（有主题，抄送，密件抄送）：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net?Subject=This is the subject&cc=hello@5dmedia.com&bcc=rock@rockstudio.com","","",5)
以下的代码可以用自动打开默认的邮件发送程序（有主题，抄送，密件抄送，邮件内容）：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net?Subject=This is the subject&cc=hello@5dmeida.com&bcc=rock@rockstudio.com&body=This is the body of the mail. This is another sentence.","","",5)
以下的代码可以发送多人：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163net;hellyun@love.com?Subject=This is the subject&body=This is the body of the mail. This is another sentence.","","",5)
以下代码可以抄送多人：
ShellExecuteA(windowhandle, "open","mailto:hellrock@163.net?Subject=This is the subject&cc=hellrock@5dmedia.com;friend@5dmedia.com&body=This is the body of the mail. This is another sentence.","","",5)

以下为ShellExecuteA的API声明相关内容：
Function Name ：ShellExecuteA
Parameters：Long,String,String,String,String,Long
Return value ：Long