MBOX files, also known as Mailbox files are supported by an extensive range of email clients as their default database. A MBOX file stores all the emails of an email client in a single concatenated structure. The emails are separated from each other with the header information of the subsequent email. The email clients, which support MBOX file, are Thunderbird, Sea Monkey, Pocomail, etc.
On the other hand, PST files are the default database format for storing Outlook database. Unlike MBOX file, PST file is only supported in Outlook.