Most of the apps I tried didn't work right, but
DataVare MBOX to PST Converter did. It was the best tool because it let me convert several MBOX files to PST files at once, instead of having to do them one by one. Everything worked great, and the folder structure, email formatting, attachments, and metadata were all saved. You can get MBOX files from Gmail, Thunderbird, Apple Mail, Opera Mail, or any other email client, and it will work fine with them. One more great thing about it is that you can use it without Outlook. Its GUI is very easy and simple, so any kind of user can operate this tool very well without facing any problem.