Posteo is great if you are looking for a solid, privacy-focussed email provider. They are not free, but at 1 EUR / month well worth the money. They offer email, address book, calendar and notes functionality using their web interface or standard protocols (IMAP, CardDAV, CalDAV). In addition to the normal transport security (including DANE server-to-server encryption where available) you can enable a variety of encryption options for additional security – they offer to encrypt your mailbox on their servers with your login password so that even Posteo can’t read your emails at rest. Or, for even more security, you can opt to have all incoming emails automatically encrypted with your PGP or S/MIME key even if the sender didn’t encrypt them. They also make sure that your payment information is not being connected to your email account so that your email account usage remains pseudonymous.
In my experience, their service is very reliable, fast, and comfortable to use. Only downside (and this is a big one, at least for me): You cannot use your own domain for Posteo! You can choose between a variety of their domains, but you cannot bring your own. This means your email address stays locked-in to Posteo, if you want to move to another provider you will get a new email address that you need to communicate to your contacts. That’s why they only get four stars from me, but if that is not an issue for you, you can’t do much wrong with them.