Re: I can't seem to permanently delete junk email.
In iOS8 on iPhone 6, I have 3 email accounts, 1 Gmail, 1 icloud, and one through my own domain. I am not able to quickly (in one step) empty the junk mailbox on the account from my domain. "Select all" > delete is what I'm looking for but it's not available. Only options are "mark all (read or unread)" and flag which don't help.
I should also add that I have tried the "hack" that is commonly recommended when Googling this topic, but that does not offer a permanent solution. In other words, the deleted emails come back.
Solution – I had mistakenly set up this email account on my computer as a POP account instead of an IMAP account. Since I never use POP accounts, it didn't occur to me to check this until I had talked with Sean who suggested that something didn't seem right with the account setup. Changing the account to IMAP still doesn't give a "Select All", "Delete All" option in the folder on my phone, but now the folder is synced to my email account on my computer and deletes messages on the phone when they are deleted on the computer. I suspect that the other preferences, like "Delete in one day" etc. will also work now.
Another thing to be aware of is that this seems to be an issue with accounts that are not icloud or gmail accounts and so it's possible to login to your 3rd party email account and use the mail interface there to edit the contents of folders like the Junk folder. Probably best to get your email account settings set up correctly, but in the mean time, this is a way to delete messages from a burgeoning folder.
Check it out – I hope this helps.