Starting with Exim 4.94 there’s some breaking changes; the docs say it the best:
Some Transports now refuse to use tainted data in constructing their delivery location; this WILL BREAK configurations which are not updated accordingly.
So if you are seeing
Tainted filename for search or something similar in
your logs then that’s the issue.
For me personally I was using
$local_part in a router to first
$domain specific aliases file, then for looking up the destination
$local_part (this didn’t need changing in my routers data
It’s worth looking at the docs to get some ideas about what expansion variables there are.
Hope that helps :)