This guide explains how to switch, in an email software/client, from a POP configuration to an IMAP configuration for the same email account. Indeed, for many reasons, it is strongly recommended to use the IMAP protocol instead of POP.

Switching from the POP protocol to the IMAP protocol…

If your email address is already configured with the POP protocol in your email software (Mozilla Thunderbird, Microsoft Outlook, etc.), it is impossible to switch to the IMAP protocol directly on the account. Indeed, the most common email software does not allow modifying an existing POP3 account and setting it to IMAP instead.

The solution is therefore to add the same email account but with the IMAP protocol in your email software/client and then to move your messages and folders from the POP account (on your computer) to the IMAP account (on the mail server).

… with Microsoft Outlook

Some versions of Outlook, such as 2016, prevent the simple addition of a second account with the same name (identical email address).

To work around this, you need to add the account from the Profiles:

And from there you will be able to add the additional account, in IMAP:

1. Set up the new email account with the IMAP protocol (the same account as the one configured in POP but this time in IMAP):
   
2. Move one folder at a time by dragging them to the inbox of the new account configured in IMAP:
   
3. Wait while the items are being transferred (this may take a long time depending on the volume to be transferred to the server); the activity is indicated at the bottom of the window:
   
   

If your POP account kept a copy of messages on the server (a discouraged option found in Outlook's advanced settings), you risk having duplicate messages.

… with Apple Mail

Deleting the email account configured with POP will delete the messages it contains. It is therefore recommended to disable the POP account instead of deleting it!

Once the POP account is disabled, refer to this other guide to configure your email address in Apple Mail, using IMAP.

Once you have finished, all you will have to do is drag the messages present in your POP account tree to the tree of the new IMAP account.