Microsoft 365 Exchange Online offers TLS support; however, it must be set up to enforce TLS. Luckily, setting up enforced TLS in Microsoft 365 Exchange Online is quick and easy, and only requires the domain name of the organization you wish to establish enforced TLS with and a valid email address from that domain To set up enforced TLS with a vendor, that vendor will need to configure their email server to enforce TLS as well. This mutual configuration assures that all email is encrypted and sent securely across the Internet.
The process of setting up enforced TLS in Microsoft 365 Exchange Online Is broken up into two steps: creating an outbound TLS connector and creating an inbound TLS connector.
You can find the steps for setting up connectors for secure mail flow with another organization in Microsoft 365 Exchange Online here.