Re: shared mailboxes, yes, correct. I've been doing a bit more research and I have to amend my previous comment: there's also Zoho, which has its own ecosystem

Re: shared mailboxes, yes, correct. I've been doing a bit more research and I have to amend my previous comment: there's also Zoho, which has its own ecosystem. Other than that, all of the standalone email providers seem to be running a standard Linux or Windows email stack (i.e. Postfix/Dovecot/Roundcube or Exchange Server). The Linux stack doesn't seem to be able to do it (well, either that or the providers don't offer it). Exchange Server might be able to do it, but it looks hard to wrangle it together with other tools. Even the Exchange hosting providers just give up and resell MS365 at that point (e.g. https://www.combell.com/en/email-hosting/exchange-hosting https://www.combell.com/en/email-hosting/exchange-hosting, https://www.combell.com/en/work-online-office365 https://www.combell.com/en/work-online-office365).