I'm not sure why you are getting the first error about the array. When I check the mailformating file that I have it only has 205 lines in it, so the error being reported on line 356 seems out of place. The other errors referencing the headers are just letting you know that output has already occurred on the page so these header calls are being ignored. These messages are only there because of the first error you are getting. Once you get past that these other errors should go away.
You should try deleting your webassist > email folder from your site root and applying UE to another page then test out that page. If you get the same error post back with a copy of the webassist folder and the page with UE applied to it in a zip archive so I can take a look.