Some questions: IMAP or POP? Are mails kept on the mail-server or downloaded and deleted from mail-server? (POP and IMAP) On IMAP are those folders created as folders on the mail-server or are they local? Have you compacted your folders regularly, because a deleted mail is just marked as deleted until you compact? If you do all your mail-savings on the server You will reach their limit.

As to create a new profile, its no use if the mail is on the server. Then you can end up downloading everything again. Better archive old mails or export them (there are good add-ons that can help you)

One account is POP. Mail is downloaded and not deleted until I delete them. And the filter message is attached (somewhere!).

Compacting I don't understand compacting. Do I have to run it on a folder & subfolders or can I run it once and it compacts everything?

Every so often the Status Bar will state that it is compacting - usually after I permanently-delete a block of messages.

Export & Import Will the folder structure be preserved? Any flavor you'd recommend?

Compacting, You dont need to do each folder by itself. under "File" you can select compact folders and it will work on all.

Well you can export it as mbox-files (as Tbirds saves them) with structure intact or as .eml singel-files (one for each email) but you then need to build your folder- structure There are other options as well.

To Read mbox-file, you need to reinstall it in a local map (temporarily) or use some other tool if you want to separate each email.

More details please - what do I use to export?

I want to keep the messages in folders and subfolders as already organized.

I use importexporttools and I haven't used mozbackup but it's recommended I answered another q and Toad-Hall has some good points Here is the link to that q. there are some links to those add-ons.