As much as I like the composer editor, line height and margins/padding are not consistent. I’ve found quite significant differences between Thunderbird, Outlook 2016 and browser view. Your HTML code is probably too “modern”. I don’t care about minor differences, but what I’ve found in my tests is not really ok. For example, relative units for paragraphs do not convert very well in Outlook. I know that Outlook is worst scenario, but many people use this piece of junk.