Even 10 euro a month per user is nothing - their salary + insurance + taxes is probably 3-4 thousand euro at least, these 10 euro are a statistical error.

We use Office365, because we need Office anyway. We initially used a hosting provider email server, but it's not trivial to get the email DNS configuration right, and you risk going in spam. Even after we got our configuration right, we still had occasional "spam" issues.