POP (Post Office Protocol) mail (Optonline is POP email) downloads a copy of each message to each device when using a standards compliant email client application (Microsoft Outlook, Apple Mail, iPad or iPhone Email app).

POP mail messages downloaded to an email client application are stored and may be accessed and managed while online or offline. Most POP mail providers allow limited online email storage.

Changes to the status (Sent, Read, Unread, Forwarded, Replied To, Filed, Flagged, Deleted) of each downloaded POP message will not automatically sync across all devices (iPhone, iPad, Mac, Windows PC, etc).

When accessing POP email only via a web browser (web mail) changes to the status of each message appears whenever accessing the same email account via web browser on other devices.

Web mail can be awkward for use on small handheld mobile devices. Email client applications provide a preferred method for managing email.

A POP message that has been deleted and email trash has been emptied via web mail or email client application will no longer appear or be available for download to other devices, but it will not remove a message that has already downloaded to your other devices.

IMAP (Internet Message Access Protocol) mail (iCloud, Gmail, Hotmail, AOL, Yahoo and others) stores messages on the mail host server when accessed via a web browser or a standards compliant email client application (Microsoft Outlook, Apple Mail, iPad or iPhone Email app).

IMAP mail providers generally allow greater or unlimited online email storage. Messages may also stored in an email client application for offline access.

Changes to the status of each IMAP message (Sent, Read, Unread, Forwarded, Replied To, Filed, Flagged, Deleted) will synchronize automatically across all devices (iPhone, iPad, Mac, Windows PC, etc.) that access the email account.

Status of message copies that are moved permanently to a local folder (on that computer only) will not synchronize to other devices.

An IMAP mail message that has been deleted from an online folder and email trash has been emptied will be permanently removed from all other devices.

For technical details about this topic visit this Wikipedia article