There's no purpose, this is most likely a configuration error on your side (i.e. in your e-mail application). POP and IMAP are protocols used for communication between the e-mail application (Thunderbird) and the mail server. Servers usually speak both POP and IMAP – and you will want to choose IMAP. From a users perspective, IMAP has two advantages: * IMAP allows for instant notification about new mail the moment it arrives on the server. POP needs to check every few minutes; depending on the settings, you may not hear about new mail for half an hour or longer (unless you hit "fetch new mail" manually). * With IMAP, the same account can be shared between several e-mail applications, e.g. on your desktop computer and your mobile: whatever you do on one device will be mirrored on the other device. POP can be used with only one application (unless you know what to do, and even then it's far from comfortable). So: # If you have two accounts for the ''same'' e-mail address, one filed as IMAP, the other as POP, move everything you need from POP to IMAP and remove the POP one from Thunderbird. #If you setup a new e-mail address, always choose IMAP over POP if asked. #If you have POP accounts without an IMAP counterpart, add a new account for the e-mail address, choosing IMAP. Then follow 1 to migrate mails from the POP account to the new IMAP one.