Maybe useful for other:
The function _Pop3Quit() send a QUIT to the server but doesn't run the function _Pop3Disconnect(), so you're never fully disconnected. A simple solution is to add _Pop3Disconnect() after _Pop3Quit()
Also _Pop3Disconnect() never reset $pop3_IsConnected to zero. So even if you use _Pop3Disconnect() you'll still be unable to connect again(Error: $pop3_error_already_connected). You need to add $pop3_IsConnected = 0 just before Return 0 in the function _Pop3Disconnect().
Func _pop3Disconnect()
If $pop3_IsConnected <> 0 Then
TCPCloseSocket ($pop3_socket)
TCPShutDown ()
$pop3_IsConnected = 0; Add this line to get a proper connection status
Return 0
Else
SetError($pop3_error_not_connected)
Return -1
EndIf
EndFunc ;==>_pop3Disconnect