Estimating the work of creating an email newsletter

I was asked to estimate the work to create an email newsletter. The work of reading email addresses and sending the mail was already in place, so all I had to do was create the HTML.
I received some visual guidance and most of the content was already present in text), so all I had to do is create one static HTML page, how hard could it be?

Boy was I wrong.

Ok, so the first HTML file I’ve sent was ok…. on my Gmail account.
…On my Office365 account it looked almost ok.
…On the Microsoft mail it looked ok’ish.
…On the MAC of the boss it looked not ok.
…On the email reader of the modern Android phone, it looked quite good, on the older Android phone, it looked like abstract art.
…On the IPhone it looked somewhat ok.
…And so on…

As webdeveloper I am used to browsers sometime implementing things different than other browsers.
But with the email newsletter the dark side of the (IE vs Netscape) past came back to live for me. Not only was it almost not possible to work CSS only so I had to fall back to the type a lot, but the differences of visualization of the HTML were surprising… and not in a good way.
This obvioulsy lead to investigating the topic more than I expected to do!