Your keyboard has a caps lock button. Please turn it off so reading what you type does not make the eyes bleed. '''NSTMP FILES''' are created when Thunderbird shuts down while a compact is occurring, or an antivirus program or other third party software made the deletion of the old mail store file impossible, so the compact aborts and leaves the new file (which may only be partial in the folder for Thunderbird to display. '''THUNDERBIRD WILL CRASH IF IT IS LINKED TO A LARGE EMAIL SERVER. OFTENTIMES THIS IS GMAIL.''' What is large? My experience is only with a few gigabytes. But it does not crash at that point, or even go slow. However I have seen reports of slowness with folders of around half a million emails. Although the slowness issues arise as a limit of available memory on the system, not a limit of Thunderbird. Perhaps we should have one. Just refuse to load folders that have to many emails for available memory, but just how do you calculate that. The system is multi tasked, so available memory values really changes thousands of times a second. So such a calculation would be out of date before it was complete. '''RECOMMENDATION IS TO DELETE THE NSTMP FILES WHICH ARE APPARENTLY AN ARCHIVE FILE. ''' They were going to be the new store file until something caused the process to abort mid way. They are not an archive. Really they are an alert that there is a problem. '''CHANGE FROM IMAP TO POP3 SERVER SETTINGS.''' I am not sure of your point here. Changing to POP only makes the entire message store local without the overheads of synchronization. A really can not see this as being helpful unless you already have significant network congestion from say a basic consumer level internet connection shared with multiple users streaming video and making video calls. It can be a method of reducing the overall bandwidth on the internet connection required. But it is a bit like letting the air out of your car tires to drive on sand. Unnecessary if you have a good road. '''ACTUAL RESOLUTION: REDUCE OVERALL EMAIL AND CREATE ARCHIVE FOLDERS.''' How does that work? Gmail uses the "all mail" folder for archives. Hence archiving mail only moves it around in the account. Reducing the size of the individual folders synchronized has benefits in situations where there is inadequate RAM installed on the machine to load the multiple folders that are open. For instance an 8Gb folder which is not all that uncommon in gmail all mail folders these days requires more that 8gb of ram to load into memory without getting bogged down in paging. (Simulated RAM) than it does about the actual size of folders. Activities like saved searches and to some extend the unified folders hold many folders open at a time, so the actual RAM requirements become quite large. Consumer laptops with a couple of gigabytes of RAM will struggle. You will often see suggestions to limit the size of the inbox, this is about keeping the memory requirement for compacting the file smaller as it is the most accessed folder and has the most deletions and moves from it. In the same "reduce memory requirements" you will see suggestions to archive in yearly archives (smaller folder sizes) and unsubscribe from the all mail folder (again remove the largest memory hog in the account) '''INCREASING CACHE FILE SIZE DOES NOT HELP.''' About the only time increasing the cache would help is if you had imap folders marked not to store mail locally. Or are in the habit of moving large numbers (in the hundreds) of emails from folder to folder in the account. The changes a queued in the cache. The automatic cache makes use of something like 1% of the hard disk drive. This is probably only ever going to help if you are running a system that is marginal in the size of disk space actually available. Despite indications to the contrary, you can not even install Thunderbird in the disk space listed as minimum. If your hard disk has less that twice the size of your two largest folders free, you will probably run into issue. Thunderbird does not cope with low disk space. Crashing is one side effect. '''DELETING NSTMP FILES IS A TEMPORARY SOLUTION.''' Certainly. But I have not seen them since about 2015 when I uninstalled my then anti virus and got a new one that worked a little faster. Now I just use windows defender. I have also upgraded my ram to 16gb and replaced my slow Hard disk (typical dell slow platter drive) with a 1Tb SSD '''CLEARLY THIS IS A LOW LEVEL ISSUE THAT IS A KNOWN PROBLEM, WHY FORUM USERS KEEP RECOMMENDING JUST DELETING THE NSTMP FILES IS PROBABLY BECAUSE THEY HAVE OTHER INTERESTS.''' No we are tired of trying to explain to people that their device is probably not capable of doing what they want to do, keeping everything they have ever received and sent in memory, or that their personal choice of security software or Thunderbird profile location is a poor one for their circumstances. It is difficult to explain to folk that that nice new cheapest computer from walmart is just not up to snuff for the job they purchased it for. This is one of those topics that has no clear cut answer simply because you only see these files when the system is under stress and really there is no real issue with Thunderbird, only the system it is being run on, or the sheer volumes of data it is being expected to manipulate in less than a second. Once the move to maildir like storage is completed. https://bugzilla.mozilla.org/show_bug.cgi?id=845952 there will be not need for compacting and no need to do anything about NSTMP files because each and every mail will be stored in it's own file. In my opinion you have identified the issue in your case in the title of the topic. "MAXXED OUT HD SPACE" I suggest you clear out any nstmp files, run a utility over your system to clear out the caches, internet temp files and generally cleans up the disk storage. I have linked to some Microsoft tools for achieving that. Then look to your largest folder in Thunderbird. Probably an all mail folder in a gmail account. Is the free space reported by windows twice the folder size? If not you really need more disk space, not a Thunderbird fix. Microsoft's disk cleanup utility is helpful in cleaning out dead wood https://support.microsoft.com/en-us/windows/disk-cleanup-in-windows-8a96ff42-5751-39ad-23d6-434b4d5b9a68 Microsoft's defragmenter is useful in speeding disk activities and opening up the free space available on the system for writing large files. https://support.microsoft.com/en-us/windows/ways-to-improve-your-computer-s-performance-c6018c78-0edd-a71a-7040-02267d68ea90 We also generally recommend folk do not have an antivirus scanning in the Thunderbird profile folders. This is generally arranged by placing an exception in the antivirus product. When manipulating files of many gigabytes in size, having the scanner lock them for around 10 minutes per gigabyte to scan them really causes a mess. So now we have discussed the generalities and my guess as to your issue. Would you like to actually provide some specific information on your system and software or will my guess as to disk space suffice for now?