One great tool I like to use is the “Remove Duplicate Messages (Alternate)” Thunderbird extension. To use the extension, you must first set the options, then you run it on a folder.
To change the options for the duplicates extension, go to Tools > Addons, find the extension, and hit the “Options” button. You’ll see the dialog shown below:
Setting these checkboxes changes the criteria for comparing two messages. The more boxes you check, the more sure you can be that any duplicates you find will actually be duplicates. I like to start by setting all the checkboxes. This takes some time, but it’s very safe.
To run the extension, right click on a folder and select “Remove Duplicates.” It will crank away for a while then show you a dialog on which you can review and delete the duplicates found. I right click on my “Mail Archive” folder, since that will compare all messages in the subfolders within the archive.
After that pass, depending on how you’ve handled you email in the past, you may need to run the duplicate remover with different options. If you do this, you’ll likely eliminate quite a few more duplicates, but you should always preview the emails in this step before deleting anything. Try turning off the following criteria, one at a time: number of lines, body, message ID and folder. I found some duplicates with minor differences in those areas due to the different ways the messages had been handled in the past. Turning off the folder criteria will find duplicates in different folders (which is nice), but be careful to keep the copy that you’ve filed properly.
Another technique I found VERY effective in clearing out my email backlog was to set up a “whitelist” in Thunderbird. Basically, I add every valid email address for a real person I’ve communicated with into an address book and set up filters to move messages around based on whether the sender is in that list.
Now, you probably wouldn’t want to add all of these email addresses to your regular address book, but Thunderbird allows you to have multiple address books. I created a new one called “Whitelist” and didn’t put a lot of effort into keeping that one neat or fully filled out. The whitelist address book will contain any email address, current or old. You probably only want active email addresses in your normal address book.
To build my whitelist, I went through my folder of unsorted mail (sorted by sender), and previewed messages from anyone I knew. For each new person, I right-clicked on the email address and chose “Add to Addressbook”. I didn’t bother filling in the person’s name, since it doesn’t really matter for this technique.
The next step is to set up a filter. Create a filter that you can run on your unsorted mail folder like the one shown below.
My filter moves all whitelist email to a folder called “Whitelisted”. It runs both when new mail arrives and when I manually run it on the unsorted inbox folder. With this setup, I can refine my whitelist. If I add some more addresses to the whitelist, I can rerun the filter until I’m left with only junk in my inbox.
After all this processing, I’ve separated my good email (from real people) from the mass mail, etc. At that point, I can move the good email to the archive and take care of the other mail however I see fit.