What would you use for business email if you were starting a tiny company today?

I’m looking at the business email market for a very small startup — probably only 1–3 mailboxes at first — and I’m curious what people would actually choose today.

The main things I care about are:

custom domain email
good deliverability
easy DNS/setup
IMAP/SMTP so I’m not locked into one app
aliases
decent security/MFA
easy migration later

not paying for a huge suite of tools I don’t need yet
The options I’ve been comparing are:

Google Workspace
Probably the obvious/default option, especially if you already use Google’s ecosystem.

Microsoft 365
Makes sense if you’re already using Outlook/Office/Teams.

Zoho Mail
Seems to come up constantly as the budget-friendly option.

Fastmail
I’ve seen a lot of positive comments about simplicity and the actual email experience.

Proton Mail
Interesting mainly for the privacy/security angle.

RelyPost
A much newer one I’ve been looking at. It seems focused specifically on small-business email and currently has a free tier for a few custom-domain mailboxes.

Full disclosure: I’m involved with RelyPost, so I’m obviously interested in understanding where it sits compared with the established providers rather than pretending I’m an independent reviewer.

For people who actually run a small business or startup:

Which one would you choose if you were starting again today, and why?

I’m especially interested in real experiences with deliverability, support, migration, reliability, and anything you discovered only after using the service for a while.