As you probably know, there is no easy way to make an e-mail from IMAP into POP3, in Thunderbird.
However, there is an easy way to do it.
Follow this tutorial
Read everything first, and then proceed.
So when you delete an account from Thunderbird, the e-mails stored will remain on your computer in a profile folder.
Open Thunderbird, right click on your e-mail account > server settings.
Check the option called “local directory” and copy the path. Open windows explorer and paste that path name.
That is the folder where your e-mails are stored. So if you delete the account, that folder will remain with all your e-mails.
Now delete the IMAP e-mail account that you want to change it to POP3.
Don’t worry, you won’t lose anything.
Recreate the account with the same address, but this time, set it to POP3 instead of IMAP. Here is how to create a POP3 account in Thunderbird.
Go to webmail, hope you know how webmail works. It is a web interface that can be accessed from yourdomain.com/webmail.
Delete all the e-mails from there.
Now, start Thunderbird. Your inbox will be empty, but don’t worry, we will fix it right away.
Right click on your e-mail account, server settings, “local directory”. If you notice, there is a new path now (a new profile, different from the old IMAP one).
Just replace the current path with the old path that the IMAP account was using. You should have it open in Windows Explorer.
You will notice that all your e-mails are in their place now.
This concludes the tutorial.
You had to delete the e-mails from webmail, otherwise they would have arrived once again in your Thunderbird client as duplicates.
Be sure to backup the folders from your “local storage” before doing all this work. And make sure you allow an IT guy who knows what he’s doing to do all this work.
If you have any questions, leave in the comments.