Which email Provider for communicating with customers?

Depends on what you need to use the email address for I think. I use Office365 for email accounts (proper accounts, with an inbox and such) and SendInBlue for marketing and transaction email. Even with marketing and transnational email you can auth your domain via TXT records in the DNS manager and have senders like [email protected] and [email protected] for example, with the added benefit of click tracking for marketing purposes.

I use Zoho for emails from my domain. They used to be free but recently upgraded to $1/month/user. That's still cheaper than $5/month/user for Google.

If you're talking about sending mass emails to people, I like SendGrid for transactional emails, and Sendy for newsletter emails.

If you only need to receive emails and don't need to worry about the "from" address when you send things out, you could use CloudFlare's new email service (currently in beta), which has free email forwarding.

I currently use ZOHO mail. There is a free forever plan for up to 5 users with some storage limits and it is super easy to set up. It has worked well for me the last few months. https://www.zoho.com

I've used Google in the past for previous startups.
Using https://fastmail.com/ now

We use lark for transaction email.

been using Fastmail for a few years now, for my own company. fast, simple, great. would highly recommend. a 3 years plan ends up at less than 3USD a user if i recall correctly.