There are plenty of services, software and platforms on which your HTML emails can be displayed. Screen resolution, UI constraints, user-resized windows are all factors that can affect the display.
Example 1: Smart phones
If Iphones resize the email to fulfill the screen, it is not the case of Android phones, which will only show the top left corner if the email is too wide.
Example 2: small reading pane
If the screen resolution is tight (eg 1024×768) or if the window was resized manually by the user, it is possible that the software does not have the necessary space to display everything. In this case, it will cut in the email and display scrollbars.
Worse still, it is possible that the images are not displayed by default, leaving large empty blocks!
It is impossible to be certain that your email will appear in full.
Make your layout to a maximum width of 600 pixels and think about what will be visible when the display is truncated. If your logo is in the upper left, make sure to provide alternate text in case the images are disabled by default.
A good tip: use the Cyberimpact’s template builder or pre-designed templates! They are specially configured so the width never exceeds 600 pixels and automatically resize your images if necessary. In addition, they are now compatible with mobile platforms.