nPopUK is your best bet, should you *not* need IMAP. QMail does work with POP/IMAP, so that's another alternative, yet QMail is no longer developed for HPC--you'll be working with releases circa early 2000's, but hey, it'll match your hardware
nPopUK still has new releases, and I use it on old laptops as well.
Either one is better than Pocket Inbox, unless your POP setup is super basic
(no smtp authentication, etc.
)
Jake