How do you delete hundreds of emails on your iPhone from your inbox that you do not need anymore without having to do it one at a time?

I have an iPhone 6S and have three separate email apps on the phone that are all working fine together. I am trying to get rid of about 300 emails that are left over in my inbox that I do not want or need. Does anybody know how to simply mark all and delete? Because every time I try to hit the mark all button A box pops up and only gives you the option of checking read or unread or flagging that’s it nothing else. This is very very frustrating so can anybody help me please. Thank you in advance