Thunderbird does support TLS and SSL encryption, but this only encrypts your email until it leaves the mail server, then it's in the clear again. Therefore you should look at encrypting your email from the time it's sent to when your recipient receives it, so no one can read your email in transit.
Here's some other good reasons to encrypt your email.
1. If email is left stored on email servers, then anyone with access to that server could also read those emails.
2. People you correspond with may not appreciate their information being sent insecurely.
3. It might be against the law in some jurisdictions to send medical and financial information without secure email.
4. It's no body elses business to read your personal email.
EncryptUs makes sending emails safe. It does this by encrypting the contents of each email so that only the recipient can read it. The email cannot be read by anyone else while it is in transit. Working with EncryptUs is simple. Once it’s installed, you still send and receive emails as you normally would. There’s nothing special or different you need to do.
EncryptUs operates invisibly in the background, encrypting and decrypting emails automatically as it manages your cryptographic keys. Plus, EncryptUs is free to use.