Messages are stored in mbox files, not as individual files. After deleting messages, you can recover the space by compacting the folder.

Thank you for your help. 1. What actually happens then in the mbox files when an individual message is deleted? I do not understand the process at all. I am ignorant about mbox. 2. Where are mbox files located in relation to the profile folders? 3. How can I recover space when the messages are all together and many I need to keep as is? 4. Which folder would I compact? and where do I do it? 5. Does this mean that it is best if I create a folder within Thunderbird where I collect messages I want to permanently delete, not keep in Trash, and then delete that particular folder completely? And if so, how do I identify that in mbox in order to compact it? 6. When messages are deleted from the Thunderbird "app", what actually happens to them to separate them from all the rest that are not deleted? So sorry if I am repeating myself, but as you can tell, I am not very IT savvy in this regard. And I certainly do not want to end up deleting by mistake things I do need.

'mbox' is an industry format. All of your message folders (inbox, sent, trash, etc) are in mbox format. These files are physically located within the account folder in the profile. You can view them by clicking help>troubleshootinginformation, scroll to 'profile folder' and click 'open folder. there you will see an imapmail folder and a mail folder. the imapmail folder is where IMAP accounts are stored and POP accounts are stored in the mail folder. alongside each mbox file is its index, e.g., inbox, inbox.msf, sent, sent.msf - where the msf files are the index files.

When a message is deleted, it is still in the mbox file but marked to not display. you can rightclick on the folder and select compact and thunderbird will then remove the marked messages and move remaining messages together to reclaim the vacated space. You can also go to settings>general and set the compact feature to periodically compact all folders. None of this prevents you from deleted messages by mistake.

Thank you so much for this level of detail. I will try it out as soon as I have capacity to do it carefully. In your view, is there a downside to compacting folders like this? If not, I am wondering why the Thunderbird software does not do this automatically anyway? And one last thing: Have you given me the instructions for Thunderbird on a Windows OS or is it crossplatform? I have Thunderbird installed on a Windows 11 computer and also on a Linux (System76 Pop! OS) computer.

- it is not done automatically, as large folders take time and some users prefer to delay compacting or to have it done less frequently - this approach works the same on all versions

Thank you again. I will mark this as solved for now.