Question up-front: Can postfix be modified to use a transport table, and will this be overwritten or could it be set up and remain in place?
I am looking to set this up for an organization, but they have a somewhat strange configuration with a split-domain having some users on one server and some on another. The bulk of their users are on a commercial email system, however there are a number of users who are not able to be set up on that system for a variety of reasons. We currently have this configuration working using mail rules/rewrites/etc on the commercial email system and a (very aged) server set up in-house that handles these other accounts and postfix transport table rules to know how to get to the other set of addresses.
Yes, it is quite a bit of a mess, but the thing right now is a desire to get off the aging server and move these users to a cloud-based standalone email machine they will access primarily through Roundcube webmail and a few IMAP users.
If the transport feature can be used, even if manually maintained as it is now, it would basically duplicate how things are set up right now.
The other fallback idea was to simply add a subdomain “webmail.xxxxxx.xxx” for these users, but it would undoubtedly cause confusion to remember the differences in email addresses.