I don't know of any way to do what you ask for without using encryption and certificates, but for your own reason you have ruled that out. Seriously, you should consider gpg or s/mime. Support for s/mime is built into Thunderbird and many other email clients. Thunderbird supports gpg via the Enigmail addon, but it's rather more possible that a correspondent would have trouble with this, unless they were also using Enigmail. It's very easy to create certificates in Enigmail, less so with s/mime, but the greater interoperability of s/mine can quickly repay the setting up effort. Alternatively, if you both have the same word processor and it provides encryption and passwords, eg Word, then you could write your missive as a word processor document, password protect it and send it as an attachment. Or you could use anything you like to create your document, then zip it with a password and send the zipped file as an attachment. But be wary. I once zipped something and forgot its password, and was dismayed at how easy it was to find and successfully use a zip password cracker.