Communication between the email servers is normally encrypted with TLS. The email files themselves are rarely encrypted. Most providers that do encryption of email are using local server managed encryption, so the email providers would still be able to access it.
For proper end to end protection you would want to setup PGP between you and your recipients, and encrypt the email before its sent.
Yeah, in 2025 doing encrypted email is a painful process. Every option is a hack on top of a 43 year old protocol.
Here is a howto from Mozilla on pgp with Thunderbird. It isn’t a pleasant process.
https://support.mozilla.org/en-US/kb/openpgp-thunderbird-howto-and-faq