Outlook has it's own renderting mechanism different to most common email providers.

Specially for elements within elements the redendering causes problems.

For example :  If your  table in question was nested in a div that had font-family:Arial; in its style, but the table did not have a font set. So tables don't inherit fonts (and perhaps other things) from their containers in HTML emails in Outlook.

Even if you set font-family: arial to table, it still wont work.

You need to specifically set the font for each td inside your table to get it right.

This is the same for almost each elements inside other font-family styled elements.

Putting the style tags in the head for table, td, p will work for Outlook, but will not work for most of the clients. In-line works well for all !