As per the title.
Must run on my PC. Must NOT send my passwords over a 3rd party server. I.e connects directly to my account's IMAP server, not via the app's server.
Must support 10 or more accounts.
Portable is a huge bonus, as I often reinstall windows and don't want to have to setup a dozen accounts every time.
Filters/rules, spell-checkers, calendar, contacts, etc are a bonus. As would the ability to look at individual accounts, but also have a unified view
Vivaldi browser started shipping a builtin IMAP mail client with its browser, in a similar vein to the old Opera mail.
It works as far as I know as a standard IMAP client, and given they are privacy oriented I'd suspect they connect directly and don't store or send your password, but haven't verified.
It is portable-ish, in the sense that it has a standalone install option, but like all Chromium based browsers encrypts some data against your operating system profile, so some data loss is expected if you reinstall the OS, though not all.
As far as I know has no restrictions on the number of accounts.
