This happens when a tool tries to load a multi-gigabyte MBOX file all at once and runs out of memory, so it throws a MemoryError or simply hangs. The fix is to work with the file in smaller pieces. Jump to your situation below or work through the methods in order.
A MemoryError or a hard freeze when opening an MBOX file means the tool tried to read the whole file into memory at once and ran out. MBOX is a single flat text file, so a large mailbox can be several gigabytes, and a converter or importer that loads it all at once will exhaust the available RAM, especially a 32-bit application. The reliable fix is to stop loading the whole thing: split the MBOX into smaller chunks (around 1 GB each) with a splitter tool and process the parts, or use a 64-bit, streaming importer that reads the file message by message rather than all at once. Closing other applications to free memory, and adding RAM, help, but splitting the file is what removes the wall.
MBOX stores an entire mailbox as one continuous text file. That is simple and portable, but it means there is no easy way for a naive tool to read just part of it, so many converters load the whole file into memory before processing. On a multi-gigabyte mailbox that can ask for more RAM than the machine has.
A 32-bit application makes it worse, since it can only address about 2 GB no matter how much RAM is installed, so it hits the wall sooner. The cure is to avoid loading the whole file at once, either by splitting the MBOX into smaller files or by using a tool that streams through it message by message.
“The mistake is treating a 12 GB mailbox like a small file and asking the tool to swallow it whole. MBOX is one long text file, so a lot of converters load the entire thing into RAM before they do anything, and the machine simply runs out. I split the file into one-gigabyte chunks first, then process the pieces, and the MemoryError vanishes. If a tool offers a streaming or 64-bit mode, use it, that reads the file message by message instead of all at once.”
The wall is loading a multi-gigabyte file all at once, so the fix is to stop doing that: split the MBOX into roughly 1 GB chunks, or use a 64-bit streaming tool that reads it message by message. A 32-bit converter is the worst case, since it runs out of memory long before the RAM does, so prefer 64-bit tools for big mailboxes.
If even split chunks fail to load, a single enormous message or attachment inside the file may be the cause. Split that chunk further to isolate the oversized message, or extract it separately, then process the rest normally. You can also submit your error to us for a tailored fix.
