An administrator configuring an outgoing SMTP e-mail connector in Exchange has a choice of how outbound e-mail will be relayed: either directly to the recipient system via DNS, or through an SMTP "smart host" or relay server. Some ISPs, whether for the source or target e-mail server, do not allow direct connection for e-mail and must have mail forwarded through a smart host at the ISP; this is usually done to prevent spamming, impersonation and other abuses of e-mail. However, many ISPs leave the method of mail delivery entirely to the server administrator, and may offer a smart host as an option rather than a requirement.
DNS has the advantage of being more direct and faster. If you choose the DNS option -- under the General tab for the SMTP Connector's properties -- note that it can fall prey to certain problems. The first is the one described above, although it is generally more of a problem with dial-up ISPs than it is with WANs. If this is the case, you will receive non-delivery reports that seem to indicate an outbound connection could not be created.
The other disadvantage depends on where your DNS is hosted: if you are relying on your ISP's own DNS, there can be resolution problems if the server is under heavy traffic, and that may result in delivery failures. On a server I administered, the ISP's DNS servers were being hammered so badly by all the other servers on the segment that I decided simply to install a local BIND server, allow two-way TCP and UDP port 53 access on the machine in question, and have Exchange resolve to that. All our DNS resolution problems vanished.
If you choose to use your ISP's smart host or another third-party smart host, some other problems can crop up. Sometimes an improperly or incompletely configured smart host can eat non-delivery reports, which can make debugging of outbound message problems very difficult. In some cases the NDRs are simply being forwarded to the administrator of the remote smart host rather than the Exchange server, where they may not be given due attention.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog for his latest advice and musings on the world of Windows network administrators – please share your thoughts as well!