I have added a html signature using a special font and size for testing
as my sogo signature.

The signature is added and the mail send as html from the sogo
webinterface, but in thunderbird or google mail, the signature font is
not used. It looks like some sort of default font is applied.

In thunderbird I checked my settings and I do allow mail defined fonts
and to show messages as original html.

The plain message text also shows the font, which is actually installed
on my apple mac.

Any suggestion or ideas what might be wrong.