How to save emails from Thunderbird as .eml

Yes

Displays a 'Save Message As' dialog where you can select a location on your local disks or network to save a copy of the message you are currently reading. You can save as .eml, .html, or .txt file types. In the Windows version, select these under 'Save as type:', in the Linux version change the default extension of '.eml' to either '.html' or '.txt' if you wish to save it in one of these other formats. (Note, the only way to open a '.eml' format file in the Linux version is to use the File > Open Saved Message menu selection.)

Source

You have to:

1. first save the email you are writing, as a draft
2. then go to the drafts folder, right click on your email > Save as... > choose .eml file type and save

I don't know why Thunderbird obliges us to this workaround.