Outlook 2016 (not sure about earlier versions) lets you send encrypted emails which should satisfy your privacy worries. The only problem is you need to have either an Exchange Online account or an Office 365 account.
You can always manually encrypt your emails if you use a decent email client using GnuPG and that will work with any email provider. The only problem is you need to exchange public keys with everyone you want to send encrypted email to otherwise they won't be able to decrypt them. While encrypting emails is the ultimate form of email privacy it is also the hardest to implement because everyone needs to make public / private key pair and you need to share around public keys. Most people end up using key servers for that.
Edit: You can use Gmail and encrypt all of your emails. It just matters what email client you use not the email provider. If the emails are encrypted by your client, Google won't be able to read them as they will be encrypted before they even reach Googles servers.
Not strictly true, over and above TLS you can set up s/mime without 365/outlook online using any version of exchange post 2003 and also any outlook client from 2003 upwards. All you need to do is set up a local Certification Authority alongside directory services to deal with enrolment/revoking certificates, there are also 3rd party S/Mime certification services available if your not up to getting down and dirty, although this config is a pain in the arse to configure you can run it on premise and it isn't exactly rocket science - I know this because, well... I set it up and run/maintain s/mime here for secure communications where TLS is not enough, this is actually the case for a lot of our tech focused clients.
Having a certification authority and the ability to roll out certification over a GPO with auto enrolment does take out any overheads later on after the initial config, it also costs nothing other than an ssl cert, so what £130 or so for 5 years. So yea, to answer the question, there isn't much more secure than a properly configured, on premises Exchange box with TLS & S/MIME, Perhaps even something like message labs in front for your filtering and continuity.