Somewhere on the DL website, perhaps in the area with their PR releases, there's a link to a page with all of the executives (IIRC called leaders) listed with their titles, about fifty names. It should be easy to identify the correct functional area for your issues. The Chris Eliott site should also show the email convention DL uses, so that if you know the name, you can figure out their email address.

With some companies, you can call the general corporate headquarters switchboard and ask for the name, business mailing address, phone number, and email of any executive by (approximate) title.

The Points Guy (major FF etc. blog) might also have lists of comtact information for airline compalint escalation people. Another source for DL specifically would be the Renes [sic] Points blog.