Postbox stores mail files in the industry standard MBOX mail format.
If your accounts are IMAP, then you do not have to export mail from Postbox. Simply set up the IMAP accounts in your desired email client and you'll have access to all of your mail within those accounts.
To export a single IMAP folder:
1) Right-click the folder you wish to export, then select Properties from the contextual menu. Click the Synchronization tab, then click the Download Now button.
2) Quit Postbox.
3) Your IMAP mail files are stored here:
~/ Library / Application Support / PostboxApp / Profiles / [your.profile] / ImapMail
Users \ [your username] \ AppData \ Roaming \ PostboxApp \ Profiles \ [your.profile] \ ImapMail
For POP and local mail folders, the mail files are located at:
~/Library/Application Support/PostboxApp/Profiles/[your.profile]/Mail/Local Folders
Users \ [your username] \ AppData \ Roaming \PostboxApp \ Profiles \ [your.profile] \ Mail \ Local Folders
The files are stored in the MBOX file format but note that #PB# files or .msf files are temporary cache or indexing files that are not needed.
From here, your MBOX mail files are available for archive or import as desired.