I think he means that office365 absolutely dominates.
1) shared mailboxes (that multiple people can manage)
2) sane auth[entication|orization]
3) retention of deleted emails
then there are only 2 players in the game: Microsoft and Google… and once you pick one of them, it makes sense to also buy into the rest of their respective ecosystems, since everything is well-integrated.
The choice then ends up coming down to whether your users prefer Google Docs/Sheets/Mail or Microsoft Word/Excel/Outlook…
did you also evaluate standalone email platforms?
is there just no commercial email provider with those features or is there not even any tech out there that solves those problems?
with shared mailboxes i assume you mean the ability for multiple people to access mails to an address and have everyone synced on responses without sharing login.
Wasting 10s or 100s or 1000s of 1000s of hours of time across an organization is stupid just for the sake of "avoiding the behemoths that dominate the market"
The fewer the points of failure, the better
Your choice of 50 unrelated, unintegrated vendors introduces 50 single points of failure to business
That's incredibly stupid - unless you truly have no choice
It introduces a huge amount of unnecessary risk to the company, and immense overhead in additional personnel (product admins, contract management, vulnerability monitoring, end user time/training, etc)
It's why multicloud is mostly not a thing: outside of a few niche cases, running cloud workloads across multiple providers is more expensive than running it all in one. Sure GCP might have something that's cheaper than AWS, and AWS than Azure, and Azure than GCP - but now you have to manage data going into and out of three providers instead of one, three separate contracts, three sets of credential groups, etc
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/work-online-office365).