I use procmail to filter my mail on the server, so I don't have to access the mail with an (imap) client first to get it sorted. I use it mostly to keep my personal and business mail separated from mailinglists. The latter wasn't working well enough and resulted sometimes in mail being delivered to the wrong mailbox.
Thanks to the procmail-lib project[1] and the procmail documentation project[2] it now works a lot better and I understand the horrible procmail syntax a bit better!
Thanks Jari Aalto for all your hard work invested in these projects!
[1] https://github.com/jaalto/project--procmail-lib [2] https://github.com/jaalto/project--procmail-doc