I have the following search criteria working very well in Gmail:

user@domain  from:/mail delivery/ || /postmaster/ ||/Undeliverable/ 

I am trying to write Goole Apps code to return the same results. Here is the code:

var thread=GmailApp.search("user@domain  from:/mail delivery/ || /postmaster/ ||/Undeliverable/ ");

I am getting different results. I am new to both Regex and Google Apps.