Is there any OSS or FOSS e-mail client, for Linux or Windows that has native ActiveSync support (not EWS)? 
I can't seem to find any, and considering it's been around "for a while", I would have thought there would be some (yes, I'm aware of all the fabulous *DAV variants, thanks).
(I mean, there are plenty of e-mail servers that run on Linux, for example, that has server-side support for it.)
@joho Thunderbird !
@AnokLola Thank you. Unfortunately, it doesn't do ActiveSync. It does *DAV for notes/memos, tasks, calendar, and contacts.
That's not bad at all, but it presents challenges for shared resources on the server.
@joho I think Evolution is your best bet https://linuxkamarada.com/en/2017/09/06/use-evolution-to-connect-to-office-365-on-linux/
@WildEnte Thank you. Unfortunately, it doesn't do ActiveSync. It does *DAV for notes/memos, tasks, calendar, and contacts.
That's not bad at all, but it presents challenges for shared resources on the server.
@harald Thank you. Unfortunately, it doesn't do ActiveSync. It does *DAV for notes/memos, tasks, calendar, and contacts.
That's not bad at all, but it presents challenges for shared resources on the server.
@joho Ah, ActiveSync in Evolution requires a plugin. evolution-ews exists in Ubuntu repos. edit: Which isn't ActiveSync but Exchange Web Services?
@harald Yes, but ActiveSync is not EWS ... it's all quite a mess 
People say "Exchange Account" or "ActiveSync", and that can mean a lot of different things, unfortunately.