Say we have three different devices, (Laptop, Desktop and Mobile device.) and you want your email to work on all three. Often the settings are confusing so we’ve explained an scenario which could help your understanding. There are essentially three different methods of accessing mail as POP3 can be broken into two different scenarios.
When using IMAP all devices will display a live representation of mail as it is stored on the server, and deleting mail from any IMAP client will delete it from the server and so will also be removed from all other clients. In other words, if an account is being accessed by multiple IMAP clients, all will always display the same content as changes made are synced between them via the server.
To avoid any confusion with this, accessing the accounts server-side using any of your webmail clients is functionally the same as doing so via IMAP, so you can treat webmail access as simply doing so via another IMAP client.
With POP3 however, there are two different scenarios as POP3 email clients can be set to either retain all mail on the server, or delete it once it is downloaded.
Assuming that at least device 1 is set to delete server-side mail after downloading, in this case with multiple POP3 devices if one device downloads a message it will then delete the server-side copy of the message, from that point on the only place the message exists is on device 1, no other devices or the webmail client will subsequently be able to download it as the message no longer exists on the server. The obvious disadvantage with this when using multiple devices is that any single message can only be downloaded to whichever device retrieves it first.
Alternatively with POP3 devices set to not delete server-side mail, when a new email is downloaded by device 1 it would be kept on the server and devices 2 and 3 would then still be able to download it, as would the webmail interface. The major disadvantage to this method of access across multiple devices is that changes made on any device do not carry over to others, so a message that has been downloaded, read, and subsequently deleted on device 1 will still subsequently show up as a new, unread message on devices 2 and 3. Additionally, as no mail is being actively deleted by the server when removed from any POP3 devices the server-side email account will store all mail indefinitely and grow in size, until mail from it is removed via the webmail interface.
Therefore, the three different methods of access are more or less suitable for different requirements, roughly as follows:
POP3 with messages being deleted: Most suitable when only one email client is ever used to access the account, and the user wants to keep the server-side mailbox as small as possible (as this stores no mail once it has been downloaded).
POP3 with messages being kept: Most suitable when only one email client is ever used to access the account and the user is happy to periodically prune unwanted mail from the server-side account, the advantage to this is that the server is functioning as a backup of mail delivered to the account.
IMAP: Most suitable when more than one email client is involved and/or the user would like the server to maintain a backup of their wanted mail, but also not have to manually prune messages from this (as mail they delete as a matter of course as being unwanted will also be deleted server-side).
In general, we recommend the use of IMAP as this offers a flexible balance of maintaining consistent mail across multiple devices while also storing a server-side backup of mail the user actually wants to keep (ie mail they have not positively deleted) while also ensuring this does not simply grow in size indefinitely (as mail they do delete is removed server-side).
We would always recommend backing up emails.