I had the same problem recently
mine did the same thing for some reason and I couldn't figure out why. I got it to delete the emails this morning. This is what I did.
click on settings -
go down to mail,contacts,calendars, press that -
press the top email account under accounts -
go down to advanced, press that -
go down to deleted mailbox and press that -
at this point there was a check mark next the trash? So, I clicked on save which moved the check mark away from trash. I moved the check mark to saved. then I went back and tried to delete my mail and it did allow me to do it. I don't know if I messed something else up in the process but at least I was able to get rid of my old emails.
Hope this was helpful to you
Sue
I just tried it again with a new mail and it worked..
