Ok, lets look at this. 1. the 4Gb limits on folder size were removed many years ago. So all that information is not irrelevant. Just an FYI, the issue was to do with how variables were defined in the C++ language and had nothing really to do with 32 or 64 bit versions. Although the issue did limit Linux and Mac to 2Gb at the time. The can not write mailbox problem as you define it is mostly the result of anti virus products locking files in the profile folder while they scan them in their typical pedestrian manner. It is for this reason this forum is full of recommendations to exclude an on access scanning from the Thunderbird profile folder. However the real issue is file contention, something Thunderbird simply can not handle, having grown up in a world where your files were available immediately and exclusively. So while I tend to reach for the antivirus card as the problem usually surfaces post update, anything that causes file contention can cause issues. So streaming backups, the profile being stored on a "cloud synced" location (something we are seeing more of), the profile not being stored locally on the system hard disk (while local network drives are supposed to work, they can be problematical and other things like NAS, and USB drives can also have built in contention issues. Again just an FYI. I have an inbox on local folders of 7.4Gb containing almost 175,000 emails. I also have a GMail account connected as IMAP which has a 6.5Gb inbox with some 225,000 messages in it. While I do see "not responding" issues for which I have no solution (or even real cause) your issues will most likely be something external to Thunderbird, but that could include spam/Scam tools injected into Thunderbird by antivirus products that have not received the message their tools are not welcome in the running program.