[freedsoftware]

Slowness and memory bloat is the main problem of Thunderbird. I don't know what the cause is, but it is frustrating that there are no full-featured open-source mail apps that have decent performance.

Kmail's UI can be fast, but its IMAP support is so horribly slow and buggy at least for me.

[tguvot]

kmail imap support was somewhat rough around 10 years ago, but it been totally stable for me ever since. i have push/idle enabled on server/client, it been working just fine. the only problem that i have it's when letsencrypt certificate rotated at night and at morning i get few hundreds popups asking to approve new certificate