On Windows, this invalid-parameter error on an MBOX file is usually a path problem: the full path is over the 260-character limit, or the name contains characters the tool will not accept. Jump to your situation below or work through the methods in order.
On Windows, 0x80070057 (ERROR_INVALID_PARAMETER) on an MBOX file is most often a path problem. Windows has a default maximum path length of 260 characters, and deeply nested export folders with long mailbox names easily exceed it, so the tool is handed a path it rejects as invalid. Illegal or unusual characters in the file or folder name (such as colons, quotes or trailing dots) cause the same error. The simplest fix is to move the MBOX file to a short location like the root of C:\ and give it a brief name such as mail.mbox, which sidesteps the length and character limits at once. Enabling long-path support in Windows is the more permanent option when you cannot shorten the path.
0x80070057 simply means a function received a parameter it considers invalid. With files, the parameter that is most often wrong is the path. Windows limits a full path to 260 characters by default, and email exports love to blow past it with nested folders and long, descriptive mailbox names.
The second common trigger is the file or folder name itself containing characters Windows or the tool does not allow, such as colons, angle brackets or a trailing dot or space. Both problems disappear when you move the file to a short path with a clean, simple name.
“Invalid parameter sounds deep, but on an MBOX file it is nearly always the path being too long. Email exports nest folders and use the full mailbox name, and you hit 260 characters before you know it. I do not fight it, I drag the file to the root of C and rename it mail.mbox, and the error is gone, because now the path is short and the name is clean. If someone genuinely needs the deep structure, enabling long paths in Windows is the permanent answer.”
C:\.mail.mbox.LongPathsEnabled).On an MBOX file, 0x80070057 is almost always a path that is too long or a name with illegal characters, not a damaged file. Moving the file to the root of C and renaming it to something short fixes both at once. Enable Win32 long paths only if you genuinely need to keep a deep folder structure.
If the path is short and clean but 0x80070057 persists, the destination drive or partition may be the problem, for example a format that rejects the operation. Try moving the file to a different local NTFS drive and running the operation there. You can also submit your error to us for a tailored fix.