David Frager wrote:

I would like to send an email to all of my contacts in a particular
category, but several have multiple email addresses.

I filter my contacts by category, highlight the specific one I want
to use and use "Action" - New message to contact.
All of the contacts are included, however, only the 1st email address
is used. Is it possible to send to all of the email addressed
defined for an individual contact?

I don't believe that's possible. You can, however, use a Mail Merge.
--
Brian Tillman