The SMTP stack provided by IIS’ SMTP svc in Exchange Server 2003/2000 changes significantly when Exchange is installed on a Windows server. Some ESMTP extensions are added to support Exchange, it gets Exchange’s Advanced Queuing Engine (AQE) implemented in phatq.dll, and the Categorizer (phatcat.dll). The mailroot path is moved from IIS’ default (\inetpub\mailroot) to \exchsrvr\mailroot. […]