Exchange Server 2003 SP2 on Windows 2003 Server SP1 releases old NDRs when physically restarted. Most NDRs are 451 4.7.1 Greylisting related. Subject emails are variously dated, some multiple months previous, and the NDRs have not been reported as released during intervening server boots, but all NDR's that are reported happen during a server boot. Main general question is whether NDRs pool somewhere that is emptied on restart and, if so, why do some ride through several restarts before appearing? Thanks.