For a start it makes sense to determine which risks you are going to be protected. The encryption will hide the contents from Resellers (people who have access to the e-mails between your computers). Electronic signature is intended to confirm that the content and the sender has not changed in the process of delivery.
If You are competent - your server obviously does not make sense - it will only increase the risk.
The classic way is to use PGP/GPG. But don't forget to use needs both You and Your contractors. And it is in very small cases, has any meaning without the use of other security measures, since at this stage to access the content of computers on which these letters write/read seems to be more straightforward.
I can still recommend the service to Proton Mail
encryption built-in, plus there are some great features such as sending encrypted messages and a contractor that works with regular mail.