Shock, surprise, horror your emails are not secure, not even particularly private. A lot of people have legitimate access to your emails.
Emails have never been secure, they were never intended to be. Your emails pass through various servers, those managing the servers, amongst others could, if they so wished, read your emails. The person managing your business server probably has full access, they have to so when you call them when you email goes down they can sort it out.
So currently your emails really are no more secure than your holiday postcard sent from Mallorca. So if you want to send your emails more securely you need to consider encrypting them.
This is one of the best systems around https://gpgtools.org/. PGP has been around since the 90's, though it's come a long way since. It stands for Pretty Good Privacy.
How does encryption work?
In this case it is what is called end to end encryption. This means only the sender and the ultimate recipient should be able to read the content.
Each person will have two encryption keys, one public the other private. You can share your public key with whomever you like. It can only be used to encrypt emails going to you. It cannot be used to read any emails at all. It's purely to encrypt emails.
You will also have a private key. Don't share this one with anybody. It will need a password to access it, so make it a good one. This key will decrypt emails encrypted using your public key.
Sound complicated but once set up it generally looks after itself, though you will have to enter your password each time you want to read an encrypted email.