Phone email messages

What could cause my iPhone to download old messages on each mail session? My phone downloads messages that were read and deleted a month or more ago. My iPad and MacBook Pro are on the same email service and they do not download the old messages, only my phone does this. It is an iPhone 6S.
Thanks in advance for any ideas or suggestions.

Make sure your email account is IMAP and not POP. Having a POP account with numerous devices being able to access mail can be a problem. (You didn't tell us anything about who your ISP is or what kind of account you have...)

Agreed. Check the access method being used, and if it's POP, change to IMAP if you can. And, verify that ALL devices are using IMAP if your account supports it. If it doesn't -seem- like it will work, check with the provider and/or look at the options through a web browser accessing the mail. Services like Gmail allow IMAP access, but you sometimes have to enable it.