POP email

POP/POP3 are essentially the same mail protocol. There are other more advanced mail protocols, of which perhaps the most common and widely implemented is IMAP.

Most mail Apps support IMAP (Internet Message Access Protocol), POP3 (Post Office Protocol v3) and other Mail protocols. Your Mail provider will provide information about the protocols that it supports.

When adding an email account to your iPad or iPhone, the device will usually detect and automatically configure the most appropriate mail protocol. Where both iMAP and POP3 are supported, iPad/iPhone will always automatically select IMAP in preference to POP3.

Add an email account to your iPhone, iPad, or iPod touch - Apple Support

When using IMAP, all your device(s) will download a copy of your email messages when fetched from your Mail providers email server. The Mail server will keep the original email messages until such time as they are proactively deleted from either your iPad or iPhone. When an email is read, it will show as such on all devices. IMAP is intended to keep mail and associated status consistent between all devices that can access the account.

If instead you are using POP3, email clients can be configured to (a) download and leave a copy on the server - or (b) download and delete from the server. As such, email messages may be deleted from the Mail Server as soon as they are downloaded by a single device.

If supported by your Mail provider, it is possible to configure a mixed environment where some devices are using IMAP and others POP3. Doing so requires considerable care - as misconfiguration can lead to unexpected behaviour. IMAP is the more modern Mail protocol that, for most domestic Users, is the preferred option.

Attempting to mix IMAP and POP3, between multiple users that can access the same accounts, is just asking for trouble. To avoid message loss, I recommend that you use IMAP - and come to an arrangement that only the individual responsible for paying the bills deletes the corresponding email(s) from the account.