"Easy" is a relative term. You can create an IMAP account alongside your POP account and create filters to move to local folders, although you may need to run the filters manually to fully populate the local folders. If there are downloaded messages from the POP account that are no longer online, you can copy them to local folders to keep them. Once you have saved all desired messages from the POP account, you can then remove it.

re :From time to time the POP account starts downloading thousands and thousand of old emails. Today it was over 29,000 messages.

Pop accounts do not synchronise with server. They get access to the server Inbox and download whatever is in Inbox. However, the pop account also has a 'popstate.dat' file and this keeps a record of everything previously downloaded so that it only downloads emails not previously downloaded. In your case it sounds like the 'popstate.dat' file has gone awol either got corrupted or is being deleted by some other program.

Check the following: If you are running any cleanup software eg: CCLeaner , Wisecleaner - then please access those programs and make an exclusion - not to scan or have access to your Thunderbird profile folders. I would also advise you do the same for Anti-Virus program. You do not want an Anti-Virus product trying to fix anything or clean up anything in your Thunderbird profile.

You are going to need to delete the current 'popstate.dat' file but this will mean yet a another and hopefully last download of everything. But you can mitigate this by doing the following.

• First, logon to your webmail account via a browser to see emails on server.
• Create a new folder - call it eg: Old Box
• Move any emails that are older than the last couple of months out of the 'Inbox' and into 'Old Box' assuming you have space and want to keep copies on server.

As pop account can only access server Inbox, it means it will not start downloading anything you put in 'Old Box'

Now to delete the current 'popstate.dat' file:

• In Thunderbird
• Help > Troubleshooting Information
• Under 'Application Basics' - half way is 'Profile Folders' - click on 'Open Folder' button

A new window opens showing the contents of your profile name folder.

• Exit Thunderbird now - it must not be running - this is important.

• click on the 'Mail' folder to see contents
• click on the pop mail account folder to see contents
• locate and delete the 'popstate.dat' file.

I've posted an image as a guide.

restart Thunderbird.

A new 'popstate.dat' file gets auto created.

The first time it should start to download whatever you have left in the server Inbox. But thereafter it should only download new emails received into the server Inbox.

AS a heads up... Imap does not work like Pop. If you need to access emails using more than one device/computer and you want to see the same then Imap is a better option providing you use it in all other accounts in all other devices.

You cannot convert a pop to an imap. You must create a new mail account and set up as Imap.

Info on Pop. Pop only has access to server Inbox to download mail. It does not synchronise with any server folder. All folders in Pop accounts are completely independent of the server. It is an independent copy of your emails stored on your computer. You can delete emails from a Pop account and still have a copy on the server OR vis versa. So the Pop account and the server can contain different emails. You can delete a load of emails off the server to create space and it will not have any effect on a Pop account.

Info on Imap Imap accounts can only display whatever is on server. Imap folders fully synchronise with server folders of same name and download copies of emails. No imap account folders can be regarded as independent of the server because they are a mirror copy of the server and synchronise with server to display whatever is on server. All folders you create will also get created on server and you subscribe to see them. Basically Imap accounts are designed to show a mirror image of whatever is on the server, so Emails should be regarded as stored on server. Any email you delete via webmail will auto be removed from the Imap account in Thunderbird. Any email you delete in an imap account will be removed off the server. Any email you move out of the imap account eg: move email to a folder in Local Folders account, this means it no longer is in imap account and will get deleted off server. So if you use any other device/computer to see email, it will be auto deleted from that imap account. Please note: many servers now auto empty Spam and Trash periodically. The time varies from server to server. Some delete emails after a week other after 30 days. You should check out what happens on your server. It is important to understand that if server empties the Trash then you may discover a load of emails you had in imap Trash folder will suddenly disappear. This synchronising of folders everytime you read, move, send, etc means there is a lot of jibber jabber going on between the imap account and the server.

Many people will use the 'Local Folders' account to store important emails maybe because they like an additional backup that's independent of server OR maybe they need to create space on server to increase available quota. In this instance, I would strongly advise that you create 'Copies' of emails to put in 'Local Folders' because if you use 'Move' (it will delete off server) and something goes awry - remember the imap account will be synchronsing will server on your choice of action - it could mean the loss of email. So please always use 'Copy to', if something goes wrong you can repeat the action.