Open the file at readmbox.com. It's read a few kilobytes at a time instead of all at once, so a 10 GB or 50 GB Google Takeout file opens in a normal browser tab without freezing your computer.
An MBOX file is every email you have, written into one file. A mailbox that's 30 GB in Gmail becomes a single 30 GB file. Many tools try to read the whole thing, or import a copy of it into their own storage, before you can see a single message. With a big file that means a frozen window, an "out of memory" error, or an import that runs for hours and then fails.
Reading, searching and filtering are free for files up to 50 GB. Above 50 GB, the first 200 messages are free to browse so you can check it works, and the one-time $29 unlock opens everything with no size cap. The unlock also covers bulk export and attachment extraction at any size.
How to Open & Read MBOX Files – 3 Easy Ways (ReadMbox, Thunderbird, Apple Mail) on YouTube
Files of tens of gigabytes. The file is streamed in small chunks rather than loaded into memory, so size stops being the problem. It's free up to 50 GB, and the $29 unlock removes the cap.
On first open the viewer makes one pass over the file to index where each message is. That's a few minutes for a very big file. The index is kept in your browser, so the same file opens almost instantly afterwards.
Open it here instead. Nothing is imported, so there's no copy to build. If you still need the mail in Thunderbird, filter by year and export smaller .mbox files that Thunderbird can take one at a time.
A normal desktop or laptop running Chrome or Edge is enough, because the file is never loaded into memory all at once. Phones and tablets are not supported.
Ready? Your file stays on your computer.
Open your .mbox file