Hm, tricky one because I don't have Outlook. Have you verified that there aren't more than one place in Outlook where fonts are configured - say, one for rich-text (HTML) emails and another one for plain text?

As far as I know, Firefox only sends true plain text emails to the default email client, so it seems unlikely that it would somehow end up with its own font size. My guess is that it's something going on with the Outlook settings that is causing this for whenever a third-party application (e.g. Firefox or any other app) triggers sending a new email message.

Does that make sense? Anyway, sorry that I can't be more specific.

Thanks for the reponse djst !

The issue appears to be in Outlook. If I change the font for new messages to be 9pt,10pt, 12pt everything is fine. For font sizes 11, 14, 18 they are converted to 12, 13.5 and 16 respectively.

It appears that Outlook has some kind of weird conversion scheme going on for the fonts used for the creation of new emails based on the mailto function.