It is only imap client !
Any POP + Imap client ?
(pop allow to have email on the desktop and never lose them nor having them not readable due to closed imap account)
Thunderbird and SeaMonkey have a setting (default?) to download IMAP mail for off-line use. I see one not very used IMAP account here has a 33MB inbox already.
Lightweight sounds good with the resources used by Dooble, OTOH, the mail client looks too simple. How it is for filters has to be seen, and what we are currently missing is OATH2 support, including MS's latest bastardization of it.
It is also a KDE app, so will need stuff ripped out.