MBOX extension is a plain text file which holds the email messages as 7-bit ASCII text and the attachments are stored in encoded format. Generally MBOX file is used for storing or organizing messages in a single text file which also stands for MailBox. The starting of each message is specified by "From" tracked by a white space character and end of each message is specified with a blank line.
The MBOX or MBOXO or MBOXRD are types of Mail Box files that are used by various open source softwares. Hence, this file extension can be opened using all these applications that generate MBOX file. For Example, Thunderbird, SeaMonkey, Evolution, SeaMonkey and more.