css in header vanishes. Responsive email no longer responsive.

css in header vanishes. Responsive email no longer responsive.

I have written a responsive html email - when I upload the html and send it, it is no longer responsive.  The html is correct. zoho seems to be inserting header info which takes away the responsiveness.

The css in the header also vanishes.

I have tried both pasting it into the html and uploading the html file.

What do you suggest?