Email rendering is the phenomenon or issue by which a same HTML email message may be displayed or rendered in many different ways by email clients and webmail services.
Each email client has is own way and capacities to interpret HTML and CSS standards used for creating email messages. Furthermore, a same client may render a message differently according to OS, browsers or devices.
Default and filtering image blocking is also a dimension of email rendering.
Email rendering issue has become even more complex with development of mobile usages - in 2013 half of emails are read on a mobile device. Email reading on mobile devices amplifies rendering issues because of:
device fragmentation (brands, models and OS)
vertical and horizontal lecture
use of email responsive design
How a single email renders differently on different mobile native email clients: