File Extension OLK File Format Microsoft Mail Mailbag Lock OLK stands for Microsoft Mail Mailbag Lock. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question Microsoft offers tools to thwart Office 2007 threats Hidden directory OLK414 How do I prevent automatic download of Gmail IMAP e-mails when the selection changes?