Thunderbird has always been my favourite email application. I've tried a few other ones but always went back because no other one is so flexible and extensible, IMHO. (And Outlook and the likes are out of question as I'm running Linux.) One disadvantage of Thunderbird is that it is still using the mbox file format by default. Basically this means that in each folder all mails are comprised in one big text file. On the other hand, when using the maildir format each mail is kept in a separate file. The advantages/disadvantages of both formats are discussed here and in this thread (obviously started by Wilders member @WildByDesign ). In my opinion, the most important advantages of maildir over mbox are the following: 1. The maildir format is much more robust. The risk of losing data is much smaller if Thunderbird or your computer crashes. 2. Incremental backups are much faster as only the new and modified files have to be backupped. 3. Scanning your mails with an AV scanner is easier, and infected mails can be removed without the risk of data loss. Those advantages are particularly important if you have a large database of emails going back many years. This Mozilla page says that maildir is still disabled by default because there are still many bugs. But hey - I've been running Thunderbird with maildir for several weeks and five mail accounts and haven't run into a single problem! On the contrary, it's my impression that Thunderbird is snappier than ever before. Besides, several of those bugs in that list refer to problems which arise when mixing mbox and maildir. Don't do that! This is what I've done and suggest: Start Thunderbird with the -p switch (thunderbird -p) and create a new profile. So you can always go back to your old profile if you run into problems! This is really a very important step! Start TB with this new profile. Before doing anything else go to Options - Advanced - Advanced Configuration - Message Store Type for new accounts and select the maildir format. Create your email accounts. If you're using IMAP just retrieve the mails anew from the server and you're done. If you're using POP3 (which is what I do for various reasons) it's more complicated: I suggest in this case that you start TB with your old mbox profile and install the ImportExportTools addon. Then go to each folder, right-click it and select Export all messages in the folder > EML format. Then close Thunderbird, execute thunderbird -p again and start your new maildir profile. Install ImportExportTools, create the (sub)folders of your old profile manually, right-click each, select Import all messages from a directory and choose the corresponding folder where you exported the messages earlier. Just in case that TB already retrieved messages from the mail servers and you have duplicates, I suggest to install Remove Duplicate Messages and remove them. In order to avoid duplicates from the beginning I would after creating the account(s) unselect Check for new messages at startup and Check for new messages every ... minutes in the account settings. Close Thunderbird and copy the correct popstate.dat files from the old profile. If all is well, you can enable those settings again. In order to import your data (filters, address books, etc.) to the new profile go to Transferring data to a new profile - Thunderbird, and meticulously follow the steps there.