Since upgrading to Lion from Snow Leopard, my iMac Mail keeps going off-line during sleep (energy saving) mode. It goes back on-line as soon as I press the "get new messages" icon but it should not be necessary to have to do this. I have checked the accounts and everything seems to be the same as it was before upgrading to Lion and, all the time that the computer is active (ie. not in sleep mode), Mail stays on-line and, as far as I can tell, the internet connection does not go off during sleep mode. Any suggestions as to what might cause this problem?