Is it possible to filter emails by subject, in a similar way to the blacklist files /etc/exim4/blacklist/by_sender, /etc/exim4/blacklist/by_hostname, and /etc/exim4/blacklist/by_ip?
A lot of the spam that I get has similar subject lines, so if I could filter by that I could greatly reduce my spam.
Yes, you can filter emails by various criteria (including subjects) using Sympl’s Managesieve filtering, although it’s at a mailbox level l.
The easiest way to configure this is via Roundcubes settings, where you can match mail and set a rule for it.
I’d forgotten about that 
I was hoping for something at the system level, but that’s a lot better than nothing.
One you’ve set the filtering once on one mailbox you could copy the cu l the config files to other mailboxes.
That’s an excellent idea, thanks 