How to Convert POP3 Mail to Imap
Switching from POP3 to IMAP is considered an email account conversion. In actuality you are creating a new IMAP email account and deleting your POP3 account. The task can be performed for both Thunderbird and Outlook email accounts quite simply. As a best practice and to save time during this process, move your email messages you want moved to the new account into folders and delete those you will no longer need.
In Thunderbird 1.5 or Newer
Open Thunderbird and allow it to load completely on your screen. Open your POP3 email account.
Select "Account Settings" in the Thunderbird Tools Menu, scroll over and select "Add Account."
Select and drag all POP3 folders you want to keep into the new IMAP email account. Simply click on each folder and drag them from the POP3 email account folders, then release mouse button. Ensure you have all folders and messages moved to the IMAP email account before continuing to Step 4.
Select "Compact Folders," in the Thunderbird File Menu. This will compact all folders that you did not move to the IMAP account.
Select POP3 account, then select "Account Settings," in Tools Menu then scroll over and select "RemoveAccount." Select "OK," then "Finish."
Open Outlook and allow it to completely load on screen. Select "Email Accounts" listed under the Outlook Tools Menu. The Email Wizard will load on your screen.
Choose "View/Edit Existing Email Account listed under Email. Select "Next." Select the POP3 email account on the Email Accounts dialog box. Select "Remove."
Choose "Yes" when prompted if you want to delete this account. Select "Add" on the Email Accounts dialog box. Select "IMAP" on the Server Type dialog box. Select "Next."
Fill in the information on the Internet Email Settings dialog box, such as your name and email address (firstname.lastname@example.org). Type in "mail.domain.com" (without quotation marks and replace "domain" with the actual domain name) into the incoming and outgoing server text boxes. Enter your user name and password, then ensure the check box next to "Remember Password" is selected.
Select your new IMAP account name by clicking the right mouse button. Select "IMAP Folders." Remove the check in the check box that reads "When displaying hierarchy in Outlook, show only subscribed folders." Select "OK." Select "Next," then "Finish."