Most of us use hotmail, gmail or yahoo etc. We don’t actually realize how many things can go into a simple email. The big corporates like Microsoft do actually help us. They provide exchange services for free (have personally had a hotmail account since university) and this replaces the mail as we edit/send/delete, producing a mirror of the information on devices which is on the server.
POP is the old fashioned way of sending information. It downloads anytime you access a device (you can imagine running a business meaning you have to check emails three times or more can be frustrating). The answer is IMAP! IMAP allows a copy to be made of the folders on the server and associated changes (edits/deletions/sent mail). Trash folders need to be created to allow mail to be deleted on your devices/webmail.
On most servers the mail is configured as POP unless this is specified in the DNS rules / firewalls. Firstly the DNS and firewalls need to enable IMAP.
There is also a support article which allow you to set up the email on your iphone: https://support.his.com/kb/kb/article/000048