I suspect this question has been asked before. I have searched the Internet and found numerous "solutions." The bottom line is that the iPhone does not provide a way to clear your mailbox!? While following up one promising lead, I managed to add 9000 messages to my mailbox! I ended up having to remove the email account from my phone.
I have a neurological disorder which results in a tremor. Consequently, procedures that require pressing the move icon while standing on one foot and rubbing my head are difficult for me to perform.
For able-bodied people, this might seem a minor issue. For me, it's a deal breaker. The next time my contract expires, I'm likely to switch to another platform.