Why does my campaign not appear correctly in Outlook?

The issue:

Older versions of Outlook relied on Internet Explorer to render HTML emails, but Outlook 2007/2010/2013/2016 use Word to display HTML content. This means that special attention must be paid to specific types of content in order to ensure compatibility in Outlook, and some content is not supported at all.

What it means:

The following elements are not supported in Outlook:

• Animated GIFs (only the first frame will display)
• Flash content
• Image maps
• Background images (including gradients and other patterns behind text)
• Images longer than 1700 pixels in height (can result in images being compressed to 1700px in height, or strange gaps and spaces)
• Tables longer than 1700 pixels in height (can result in strange gaps and spaces because Outlook inserts a page break)
• Manual line breaks that have been inserted into a column or table by repeatedly pressing "Enter"
• Forms within the email body
• Float or position commands
• CSS (limited support only)
• Margins and padding

What we do:

We work to ensure the default content in all the templates we provide is setup to be compatible with Outlook.

Additional resources:

Additional detailed information from Microsoft can be found here:
http://msdn.microsoft.com/en-us/library/aa338201.aspx