Turn the text file into a DB file, by makemap. The key is the address which the mail is addressed to, while the value is the new address. Note that all domains which appear in the keys much be part of class 'w' or class 'VirtHost'. Feel free to use them in your own mail systems. No official addresses or namespace.
Local domain is domain.fake.uk.
Only a few users. Users have POP or similar mailboxes at different domains, all of which should be collected to the local system. One mail server, several clients. There are a couple of support files needed, as well. A sneaky way of doing things which only works if your mailbox provider will handle it.