This is because different styles look different in every email host (yahoo, hotmal, gmail etc.)
This is often cause by users copy/pasting styled text into the email editor.
Note: Spaces and tabs are the worst for making things look out of place, each email editor displays these differently so keep this in mind when designing your campagin.
The easiest way to fix this is to create everything inside tables, as every email host displays them. [This is why we reccommend Layouts or Templates as they already have these]
If you are still having trouble after you have tried using tables, feel free to contact us and we will be able to assist you.