GNUMail stands as an open-source, cross-platform email client that finds its primary development focus on Mac OS and UNIX-like systems. Powered by GNUstep, this versatile email client offers robust support for POP and IMAP accounts. Moreover, GNUMail comes equipped with comprehensive filtering options that apply to both incoming and outgoing messages. It boasts two native file formats: Maildir, where messages are neatly organized into subdirectories, and Berkley MBOX, a structured email database file. In terms of security, GNUMail prioritizes data protection and employs GnuPG email encryption for enhanced security measures.
While it's a commendable platform, GNUMail unfortunately lacks HTML format support. Additionally, the emergence of more advanced platforms like Outlook has led to a decline in its user base. Consequently, both organizations and individuals are increasingly transferring their mailbox data from GNUMail to Outlook. However, such a migration requires the assistance of a robust migrator tool. The most recommended email converter is Kernel for MBOX to PST.
Below are some guidelines you should adhere to when converting GNUMail's MBOX files to PST format