Which option does Fastmail fail on?
Consider the following scenario: Alice and Bob jointly manage a mailbox sales@acme.co. An email from a customer arrives and Alice replies to it.
With a real shared mailbox, Bob should be able to see the e-mail Alice sent, as they both have the same view on the shared mailbox. IIUC, with a simple distribution list this is not the case (the sent e-mail just resides in Alice's personal mailbox).
For comparison, here's the MS documentation: https://docs.microsoft.com/en-us/microsoft-365/admin/email/c...
I'm a big proponent of open standards, so if anybody on HN knows e-mail providers that can sanely do this and store your data within the EU (Fastmail says they don't [1]), I'm definitely interested to hear about them. We would also have to find decent alternatives for other parts of MS365, though (file sharing and single sign-on (currently our users have different logins for various systems, causing them to write their passwords on sticky notes...)).
[1] https://www.fastmail.help/hc/en-us/articles/1500000280221#ph...