Is there a way to transform the imap email that is already downloaded to my computer to pop email so I don't need to re-download 16GB of emails?

No. IMAP mail accounts are completely different. An Imap mail account will see a remote view of any subscribed folder on the server. Folders synchronise to maintain a copy of what is on the server. Usually, only headers are downloaded and when you click on an email to open it, the rest of the email is downloaded into a temporary folder to facilitate viewing. These emails are not stored in Thunderbird.

It is also possible to download copies of what is on the server into Thunderbird imap mail account folders, but these folders will still show what is on server, so if you delete an emial via another device eg: phone or via webmail, then that email will disappear from Thunderbird. You can tell if emails are downloaded/copied to Profile by going into offline mode and seeing if you can still read emails.

Can you still read all emails in offline mode ?

POP mail accounts can only access the server Inbox. Emails are downloaded from pop server and stored in Thunderbird pop mail account. It is usual for downloaded emails to be auto deleted off the server. However, you can modify Account Settings to 'keep messages on server' if required. Please note that pop servers may not offer the same amount of space as imap servers as it is expected that you will download and backup your emails and therefore not require the server in the same way as an imap mail account.

So bottom line is this. You cannot swap imap to pop nor vice versa simply by changing server settings. They are two different things in every way.

If you want Pop then you have to create a new mail account and make sure it is created as a pop mail account. But before creating a pop account please read on.....

Regarding emails in imap account. If you can go into offline mode and read email content ok, then whilst in Offline mode, select to 'Copy' emails into some suitable folders in 'Local Folders' mail account. These emails are not connected with imap, they are now stored on your computer in 'Local Folders' mail account.

NOTE: If emails are left in server Inbox, a pop mail account will download them. Options: 1: Remove all emails off server because you have a copy of them in 'Local Folders' mail account. In which case delete email in imap mail accounts folders and then compact to remove all traces. Then go back to online mode and they should get deleted off server.

OR 2: If you have got all emails copied into 'Local Folders', but for some reason you want to keep them also on server - so you can read them via webmail. Move all emails from the imap 'Inbox' to other imap folders. If emails are not in server Inbox, then the new pop mail account will not see them so cannot download them.

Create new existing mail account and set up as pop. Unsubscribe from seeing imap folders. Remove imap mail account.

Thanks!

With imap I currently download everything (so I can see all my emails in offline mode).

Since I can't convert my imap mail to pop mail then what I am thinking is to create a new pop account, let it download all email, and then delete the imap account from my computer. Would that work?

Yes, but it is exactly what you were seeking to avoid in your original question: "From imap to pop3 without re-downloading email".

Yes, but I would also prefer to have all my email in my inbox as I have it now, instead of Local Folders.

How about this:

1. I copy my imap messages to Local Folders 2. I delete everything from the imap account (so everything will be deleted from server as well) 3. I create a POP account (there is nothing to download since server is empty) 4. I copy the emails I saved previously to Local Folders to the inbox and sent folders of the new POP account I created.

Would that work (particularly step 4)?

4. I copy the emails I saved previously to Local Folders to the inbox and sent folders of the new POP account I created.

You could copy them back again into pop account, but is it worth moving all your emails again? You now have them out of the imap folders which are synchronising with server folders and stored on your computer. Moving them again will not change how they are stored nor your access to them, it will just involve your time so it's up to you.

I would not advise you copy the emails into the pop Inbox. The Inbox should ideally be used as an inbox for incoming mail and not as a general storage.

I would advise you to create new folders in the pop mail account to organise your emails. Emails are written to an mbox file - not a folder - written one after the other in the order downloaded. You only end up with all your eggs in one basket - to coin a phrase. Suggest you have a read of the info at this link - it is all about keeping a healthy Thunderbird and reducing risks. http://kb.mozillazine.org/Keep_it_working_(Thunderbird)

Might be worth backtracking to check why you wish to swap from IMAP to POP. Many of us see that as a retrograde step. ;-)

If you're simply trying to get your messages off the server to free up space then you don't need to change to POP. Just move it all into Local Folders.

The reason I prefer to leave all received email (apart from spam and trash) in the inbox is because I can easily find what I need by using the "Filter these messages" input box in the toolbar and this only works for a single folder. I find this way much faster than using search.

But maybe there are better ways to go about it?

Well, I file things by context. I create a folder for each correspondent or topic or project. If it's from my bank, it'll be in the "bank" folder. If it's about my pets, it'll be in the "vet" folder. At work, it's a three-digit number representing the project. I have tools that let me locate a folder very quickly by typing in part of its name, much like the autocompletion of email addresses when you compose a message.

Not sure how this impacts on POP vs IMAP and why you now want to change. As Toad-Hall described, you can't be sure that your email is actually on your computer. So the only way to be sure you have it all is downloading it "again".

In your place, given that you have embraced the power of computer searches, (whereas I'm happy to put in a bit of extra work for myself) I think I'd still move all my messages out of the Inbox. You could create a folder named "Read" or similar to keep them in.

I want my Inbox to show me what needs working on right now, that is, all the new stuff. All the old stuff is safely out of the way. I consider the Inbox a somewhat unsafe place.

I know that Thunderbird works by keeping all the messages in a folder as one big file. I'm apprehensive about the wisdom, practicality and efficiency of searching that big folder. Yes it is indexed, but every message you add increments the index file too. At some point the sheer amount of data will start to stretch the capabilities of the program, your computer's memory and its OS. By spreading my data into many smaller folders, I am unlikely to reach or even approach any such limits.

As an aside, I was struggling to work with a 4 GB log file; none of the editors I normally use would open it. I had to chop it into multiple smaller files. I really don't know how well Thunderbird is managing a file that is probably bigger than the amount of free RAM on your machine. There could be an awful lot of paging going on. :-(

I was considering to move old mail to Local Folders but I had some issues with Local Folders when I tried to use it for old sent mail:

https://support.mozilla.org/en-US/questions/1180456

I don't know if you or anybody else can help me with that issue, because I got no replies so far.

Assuming those issues are resolved, is there a way to automatically move email older than X days from inbox to a "Read" or "Old" directory in Local Folders?