File with a .MBS extension contains e-mail messages which constitute the content of a specified folder within Opera Mail account, stored in a format similar to Mozilla Thunderbird files. There is no possibility to read them directly from a drive, instead they have to be conversed (with such tools as Aid4Mail or ABC Amber Opera Converter) into formats available for other mail software or text documents.

Opera Mail is a mail program built into Opera Internet browser. Each catalogue, for instance the inbox, send messages or drafts, is stored in separate MBS files.