https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=2451

aplicar o patch no debian:
apt source claws-mail-multi-notifier
cd claws-mail-3.17.8

descarregar o patch e colocar na pasta. depois:
patch -p1 < 0001-Notify-on-unread-messages-also-in-the-systray.patch
dch -n "Aplicar o patch de https://www.thewildbeast.co.uk/claws-mail/bugzilla/show_bug.cgi?id=2451"
dpkg-buildpackage -us -uc

https://unix.stackexchange.com/a/324699