Proper way to inject css

[pseudomyne]

Hi,
I fail to inject some css in my emails.

I have (setq org-msg-enforce-css org-msg-default-style)
Preview is formated correctly (C-c C-e), but once sent to gmail, it won’t render as such.

I have tried to play with some org export options like org-html-head-include-default-style t / nil and org-html-html5-fancy t / nil with no result.

I also tried to inject external css with #+HTML_HEAD: <link rel="stylesheet" type="text/css" href="/home/me/style.css"/>,
Again it's ok within the preview, but won’t be sent through mail.
What options do I miss here ?

@mclearc : would you mind sharing your (whole) config options ?