Usually either the app you're using doesn't support MBOX, or the file is too big for it to load. Try opening it at readmbox.com first. It needs no import and reads huge files in small pieces. If that doesn't work either, go through the causes below.
Windows has no built-in program for .mbox files, and double-clicking one just asks which app to use. Outlook can't open MBOX at all. It only imports PST and CSV. Notepad will technically open the file, but you'll get a wall of raw text, or it will freeze on anything large.
Fix: use something that reads MBOX: this browser viewer, Thunderbird, or Apple Mail on a Mac. For Outlook, see how to get MBOX mail into Outlook.
Google Takeout delivers a .zip or .tgz. Programs can't read the .mbox while it's still inside the archive.
Fix: unzip the archive completely, then open Takeout/Mail/All mail Including Spam and Trash.mbox. If Google split your export into several archives, unzip all of them.
When you export a mailbox from Apple Mail you get something called Inbox.mbox that is actually a folder. The mail is in a file called mbox (no extension) inside it.
Fix: open the folder and use the file named mbox. Drag it onto the viewer. More on MBOX files on a Mac.
A 20 GB Takeout file makes many programs freeze, run out of memory, or sit on an import for hours. Nothing is wrong with the file. The app is trying to load or copy all of it.
Fix: use a viewer that streams the file instead of loading it. This one reads tens of gigabytes a few kilobytes at a time. How large files are handled.
If a Takeout download was interrupted, the zip may be cut off, and the .mbox inside it is cut off too. Signs: the unzip shows an error, or the file is much smaller than your mailbox.
Fix: download the archive again from the Takeout email link (it only works for a limited time), or create a new export.
Some exports have a different format with a similar name: .pst (Outlook), .olm (Outlook for Mac), .eml (single messages) or .emlx (Apple Mail's internal format). If this viewer opens your file and reports no messages found, it's probably one of these.
Fix: check where the file came from and which format that app actually exports.
What is an MBOX file? on YouTube
Most often the app doesn't support MBOX (Outlook, Windows), the file is still zipped, or it's too large for the app to load. On a Mac, an Apple Mail export is a folder, and the mail is in the file named mbox inside it.
No. Outlook only imports PST and CSV. Export the messages as .eml files and drag them into an Outlook folder instead.
Open it in desktop Chrome or Edge at readmbox.com. Nothing is installed and nothing is uploaded.
It's probably not an MBOX file (for example a PST or OLM with a misleading name), or the download was cut off. Re-download it, or check which format the source app exports.
Ready? Your file stays on your computer.
Open your .mbox file