MBOX is a file format used by a web based client i.e Thunderbird, Apple mail, Mozilla mail, etc. to store the mailbox items. It uses POP3, SMTP, IMAP protocols to preserve the email services. Also, it is used to maintain the newsgroups, RSS feed, etc. PST (personal storage table) is a file format which is used to store data in Microsoft Outlook. Users find a regular improvement & advancement in MS Outlook and due to which the number of users are also increased there. Many users find MS Outlook better than Thunderbird and hence wanted to switch to that. Check out the here the main reason of MBOX to PST conversion and manual process to convert the files from MBOX to PST.