Is there any way I can set sorting rules for incoming mail on the server itself?
I have a few different email clients I use at different times, and it would be great if I could filter emails into folders as they come into the server so regardless of the software I’m using, they’re already sorted.
As @openletter says, just log in via the webmail interface (Roundcube) and set up whatever rules you want there. They will be applied on the server and you will see that reflected in your IMAP clients.
I have several filtering, sorting and forwarding rules set up which work perfectly for me.