POP3 accounts did not work on 21.12 it looks like it's fixed.
https://invent.kde.org/pim/kdepim-runtime/-/commit/943d0ef8c91b80671d3c519042722772247259fa
KDE PIM POP3 fixes
Already applied, since it was announced on the upstream ML:
https://mail.kde.org/pipermail/distributions/2021-December/date.html