Just use PGP, but keep in mind headers won't be encrypted. If it doesn't fit your bill, just give up and use other methods of communication that aren't email. Email just wasn't built for this sort of purpose in mind.

Friendly suggestion: Please add more context and details to your question.

What’s your goal? What’s your threat model?

Example: If jane@gmail.com sends an email to john@gmail.com, there’s not much risk that John’s creepy neighbor would MITM. But the payload (email) is not end-to-end-encrypted, i.e. Google can access the plaintext.

For sensitive conversations: Please use Signal instead.

7-zip a text file with a password then email it a friend and tell them the email sentence password on a voice phone call. e.g. "The quick brown fox jumped over the lazy dog." and explain the capitalization and punctuation. It's not a high level of security but simple enough for anyone I have ever emailed to figure out. Save the sentence used with a specific friend in your password database under a nickname for them and only use it with that specific friend.

  attachment: muffin-recipe.7z

"Here is Grandma's old muffin recipe. Please do not share it with Betty Crocker"