In theory consolidation of SMTP services on Domino is the cleanest configuration, however, in both configurations the MTAs are dependent on each other.

Since Domino is the migration direction, consolidation of SMTP services on Domino is intuitive. Going with this solution implies that the amount of mail passing through this relatively inefficient route will be systematically reduced and ultimately eliminated when migration is completed. The negative impact of any problems related to using multiple MTAs will be reduced and these problems will ultimately be eliminated when migration is complete because the cc:Mail MTA will no longer be needed. In any case, consolidation of SMTP gateway services on Domino is more efficient than consolidation of SMTP gateway services on cc:Mail.

Enter the intelligent front-end host
The best solution is to maintain independent SMTP services for cc:Mail and Domino during migration. The cc:Mail system can continue to use its existing gateway and Domino users use the SMTP MTA to send and receive mail from the Internet. At the same time routing Domino mail to and from the Internet via a cc:Mail-based SMTP gateway can be more problematic than going the other way. The only problem with using separate SMTP services for the two systems during migration is that inbound mail can only be routed to the SMTP gateway for cc:Mail or to the SMTP MTA. Enter the intelligent front-end host.

An SMTP host that can route mail based on the user ID (instead of just the domain) can send mail for Domino users to the MTA and mail for cc:Mail users to the cc:Mail SMTP gateway. This removes the dependency of the MTAs on one another so that Internet mail is autonomous for both systems. With an intelligent front-end host, inbound messages for cc:Mail users are routed directly to the cc:Mail SMTP gateway while inbound SMTP messages for Domino users are routed directly to Domino. Figure C shows a diagram of how that works.

FIGURE C


Here's a diagram of a Front-End Host Solution. Roll over picture for a larger image.

The intelligent front-end host is the most efficient alternative and it eliminates dependency of SMTP and cc:Mail MTA services. The intelligent front-end host also provides a logical load distribution (it does not impose the burden of Internet mail for cc:Mail users on Domino or vice versa). The core technology of the front-end host solution is essentially sendmail, which is known for its scalability and reliability. Using a sendmail (or derivative) as an SMTP relay for inbound mail improves both capacity and reliability is what I'd recommend for use with all SMTP gateways including the Domino SMTP MTA.

Directory synchronization
In order for the front-end host to route mail to an appropriate MTA or gateway, it must know which SMTP domains and user IDs go with which MTA or gateway. This means that some type of directory must be used. If a directory is used, it should be synchronized with the Domino NAB. We use Netscape Directory Server and Messaging Server to built front-end host solutions and synchronize the NAB one-way with the Netscape LDAP directory (using our GSS LDAP Synchronizer). Domino isn't used as the LDAP server because it doesn't support storage and retrieval of arbitrary data.