Access denied when SendKey.SendInput on server

I try to send emails from a Windows Server 2008 R2 using the shortcut CTRL+ENTER in Thunderbird.

If I am connected over remote desktop connection, emails can be send.

However when I minimize the remote desktop connection, I instantly receive an access denied error:

System.ComponentModel.Win32Exception (0x80004005): Zugriff verweigert
   bei System.Windows.Forms.SendKeys.SendInput(Byte[] oldKeyboardState, Queue previousEvents)
   bei System.Windows.Forms.SendKeys.Send(String keys, Control control, Boolean wait)
   bei PLS_CL.BTC.Logic.clsALG_Mailinfo.RunJob()

My code to send keys

SendKeys.Send("^{ENTER}")