Email and encryption...
As mentioned by @Ccharlie Clark, most SMTP servers support TLS, so that the data transmission is encrypted and less likely to be caught by someone sniffing the packet stream in the middle somewhere.
Encrypting the message contents, at rest(1)? Different animal entirely. I'm speaking here as both a mail admin and PKI admin, and I can tell you that configuring an entire company's worth of user accounts to use encryption is a very tall, and VERY expensive order.
First you'll need to ether A) Purchase a public signing certificate, which most certificate resellers will either want 5 digits for if they'll do it at all; OR offload your chain of trust for the domain to them and pay a per user, per year fee for it;(2) B) run your own publicly accessible CA for certificate validation and get one of the root trusts to sign off on it (not easy OR cheap); AND train your support staff, users, and probably random members of the public, vendors, and whoever else whines about how to react now that the emails are all encrypted using their very own personal certificate.
Oh yeah, and since you are the SME for implementing it, you are now the point of contact for EVERYTHING email related that breaks, even if (or especially if) it's not actually related to the encrypted email chains.
(1) Defining "at rest" here as sitting in an outbound, transitory, or inbound mail spool/datastore/mailbox.
(2) [RedactedCo] has in excess of a thousand users; $20 /user, per year; that's a lot of money for something that'll likely rarely get used unless it's forced on, which causes even more headaches on the receiving side.