There is only one file type that uses the .MBS extension:
Opera Mailbox File
Mailbox file created by Opera Mail, an email client built into the Opera Web browser; contains email messages for a specific mail folder, such as the inbox, drafts, or sent messages folders; stored in similar file format to the Mozilla Thunderbird mail folders.
The Aid4Mail utility can read MBS files and export them into other mailbox formats, including standard .MBX files and Outlook, Windows Live Mail, and Thunderbird proprietary formats.
ABC Amber Opera Converter can also convert MBS files into human-readable formats, such as .TXT, .PDF, .RTF, .HTML, and .DOC files. The MBS2MBX tool can also convert MBS files from Opera version 7.x into .MBX Mbox files. However, it may not translate some messages correctly.
Updated: December 9, 2017