My employer has asked for a responsive email template that our Project Managers can send to new customers from the email client of their preference.

Our PMs use either MS Outlook, Webmail, or Thunderbird to contact our clients.

I am very new to responsive email so this has been a mountain of a task and I have yet to find anything that displays consistently when sent from each of these clients and actually works correctly when received by our top 6 (Gmail, Aol, Hotmail/, Yahoo, Webmail, MS Outlook).

This is the most recent iteration of our template.

It displays nicely being sent by MS Outlook & Webmail but is an utter nightmare coming out of Thunderbird, which wipes the styling from the text in the darker blue box and displays it as tiny default text.

Completely aware that the issue could be user error...

But is it actually POSSIBLE to get consistent, responsive results sending from these three clients or am I on a fool's errand?