How do I choose "System Font" in the font picker?

Solution 1:

It may seem odd, but you can't find the "System Font" in the Font Picker. However, you can restore the default "System Font" setting for the message list (and then, if you like, use the Font Picker to adjust only its font size). Here's how.

[NOTE: I've tested this method successfully in the "Big Sur" and "Catalina" versions of macOS.]

Easy way: the "defaults" command

Quit Mail.app if it's running, and then open Terminal and execute the following command:

defaults delete -app Mail MessageListFont

The next time you launch Mail, you'll find the message-list font has reverted to the system default. Enjoy!

A less elegant alternative, if needed

If for whatever reason the technique described above doesn't work — and you're desperate — you can try this "brute force" method instead.

Quit Mail.app if it's running, and then open the following file in a text editor (such as BBEdit):

~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist

(To be extra-safe, make a backup of the original .plist file before editing it.)

Once open, find the "MessageListFont" key/value pair. It will look like this:

    <key>MessageListFont</key>
    <data>
    YnBsaXN0MDDUAQIDBAUGBwpYJHZlcnNpb25ZJGFyY2hpdmVyVCR0b3BYJG9iamVjdHMS
    AAGGoF8QD05TS2V5ZWRBcmNoaXZlctEICVRyb290gAGkCwwVFlUkbnVsbNQNDg8QERIT
    FFZOU1NpemVYTlNmRmxhZ3NWTlNOYW1lViRjbGFzcyNAKgAAAAAAABAQgAKAA1hUaG9u
    YnVyadIXGBkaWiRjbGFzc25hbWVYJGNsYXNzZXNWTlNGb250ohkbWE5TT2JqZWN0CBEa
    JCkyN0lMUVNYXmdud36FjpCSlJ2irba9wAAAAAAAAAEBAAAAAAAAABwAAAAAAAAAAAAA
    AAAAAADJ
    </data>

Delete the entire key/value pair, and then re-save the file. (If it seems you're not able to do this directly, try using "Save as…" to save the edited version "on top of" the original version, replacing it.)

The next time you launch Mail, you should find that the message-list font has reverted to the system default. Congrats!