Subject: OU and subdomain best practice
For you scenario best would be to create multiple global domain document rather than addins the domain in “Alternate internet Domain Aliases”, because even with this setting all you are saying is the new alternate domain is alias to old one.
Excerpts from Lotus Admin Help
Inbound address lookup when the Domino Directory contains multiple Global Domain documents
After Domino accepts a message, the Router attempts to match the recipient’s Internet address to an entry in the Domino Directory. When looking up the recipient in the Domino Directory, if the domain suffix in the address matches an alternate Internet domain aliases defined in a Global Domain document, and no Person document includes this address, the Router performs a secondary lookup. In this secondary lookup, the Router pairs the local part of the address with the domain suffix of the primary Internet domain specified in the Global domain document.
For example, a server receives a message for craig_bowker@acmewest.com. The Router searches all of the Person documents in the Domino Directory for this Internet address, but cannot find a match. However, in the Domino Directory, there is a Global domain document that includes the domain suffix acmewest.com as an alternate Internet domain alias. In this same Global Domain document, the primary Internet domain is acme.com. After the primary lookup fails, Domino performs a secondary lookup, using the address craig_bowker@acme.com. Domino performs secondary lookups only if the Router is configured to perform fullname, or fullname, then local part lookups.
In cases where the Domino Directory contains multiple Global domain documents, and a secondary lookup is required, when replacing the domain suffix in the original address with the domain suffix of the primary Internet domain, the Router only considers Global domain documents that list the alternate Internet domain alias. That is, Domino always replaces the domain suffix from within a given document; it never replaces an alternate domain listed in one document with a primary domain from another document.
To prevent the Router from using domain aliases when looking up addresses, do not include alternate Internet domain aliases in a Global domain document. Instead, create multiple Global Domain documents, each specifying a different primary Internet domain.