At Postbox, we always recommend that you setup your accounts as IMAP instead of POP for all the reasons outlined in this article:
This article will outline how to switch an account from POP to IMAP. Note that you cannot convert an existing account from POP to IMAP. The account must setup as IMAP along side the POP version, and then content must be copied from the POP account to the IMAP account.
Setup the Account as IMAP
Go to the File menu and select New > Mail Account. Enter your name, email address, and password, then use the automated suggestions for IMAP, or enter the correct IMAP settings in the account setup window.
Organize Your Accounts
At this time you will have your account setup as both POP and IMAP within Postbox. Since we'll be copying messages, it will be easier if both are organized within the same Account Group. Organize your accounts in the accounts pane via drag and drop so that they appear in the same group, then Restart Postbox.
Next, we'll need to rename the accounts, so that you can differentiate between the POP and IMAP Accounts. Go into Preferences/Options > Accounts and then clearly label your POP account with "POP" and your "IMAP" account as IMAP. Close the Accounts Panel.
Copy Messages and Folders
What you will most likely find is the following:
* Your POP Inbox may have more messages than your IMAP Inbox, depending on how you had the POP account setup in Postbox.
* Your POP Sent folder may have more messages than your IMAP Sent folder, because only POP Inboxes sync with the server, not Sent folders.
* You will have storage folders in your POP account that are not in your IMAP account, because only POP Inboxes sync with the server, not storage folders.
Select the POP account Inbox. Next, right-click the IMAP Inbox and select "Open in New Window". This will allow you to compare the POP Inbox vs. the IMAP Inbox.
If you see a message in your POP inbox that is not within your IMAP Inbox, then right-click the message, select "Copy To..." and then navigate to the IMAP Inbox.
Repeat this process until the Inboxes match, and verify that the content in the IMAP Inbox is loading okay.
For the Sent folder, since this folder is not synced, you can copy the Sent messages from the POP account to the IMAP account without risk of duplication.
Within the Sent folder, select about 500 messages, right-click and then select "Copy To..." from the popup menu, and then navigate to the IMAP sent folder.
Repeat this process in batches of 500 at a time, until all of the sent messages are copied over.
Repeat the process used for Sent folders with the Archive folders.
For POP storage folders, you can copy over entire folders at one time. Select the Account Group that contains both accounts, then in the Folders Pane below, you'll see the storage folders for both accounts. Expand the storage folders so that you see both at one time. Drag one POP storage folder to the desired location within IMAP storage folders.
WAIT for the copy process to fully complete. Do not check for mail, do not compose messages, just let the COPY process complete. Then inspect both the source and destination folders to ensure that the message counts are the same.
Repeat this process for each POP Storage Folder.
Creating a Backup Before Deleting the POP Account
Prior to deleting the POP account, it would be wise to make a backup, just in case.
Quit or exit from Postbox so that it is no longer running.
Then navigate to:
~/Library/Application Support/PostboxApp/Profiles/[your profile]/Mail/[your POP account]
(Note: In macOS the Library folder is hidden. To access it, open the Finder, hold the Option key, then select Library from the Go menu.)
Users\username\AppData\Roaming\PostboxApp\Profiles\[your profile]\Mail\[your POP account]
Note: The AppData folder is folder is a hidden folder; to show hidden folders:
Windows Vista / 7 - Open a Windows Explorer window and choose "Organize → Folder and Search Options → Folder Options → View (tab) → Show hidden files and folders".
Windows 8 and higher: Open a Windows Explorer window and choose "View Tab → Options → Change Folder and Search Options → View Tab → Show hidden files and folders, then click the Apply button".
COPY (don't move) the POP account's storage folder to the desired backup location.
Delete the POP Account
Once you have verified that the Inbox, Sent folder, and Storage Folders are all within the IMAP folder as you expect, only then should you delete the POP account from Postbox.
Right-click the POP account, and then select "Settings..." from the popup menu. With the POP account selected in the Accounts Preferences Pane, click the minus (-) button in the lower left.
When prompted as to whether or not you should save data, select "yes".