In mail.app:

Edit > Save as...

Format: Raw Message Source, this will save as EML.

You can also use Rich Text Format (will save as .rtf). Then you can use GNU's UnRTF. Can be installed with brew

brew install unrtf
unrtf --html input.rtf > output.html

In mail.app menu:

Edit > Save as...

Format: Raw Message Source, this will save as EML (some form of html as mentioned by Jasper Duizendstra's answer and comments).

You can also use Rich Text Format (will save as .rtf). Then you can use GNU's UnRTF. unrtf can be installed with brew like this:

brew install unrtf

Here's how you'd use it

unrtf --html input.rtf > output.html