Email Migration Guide
By To move Pegasus Mail to Thunderbird, copy your user folder from inside C:\PMAIL\MAIL and load it in a converter that understands PMM and CNM. Pick Thunderbird as the output while the client is closed and run it. The folders appear under Local Folders with their nesting and dates. If you prefer files you can keep, choose MBOX instead and import each one with the ImportExportTools NG add-on by right-clicking Local Folders. The client itself has no Pegasus import, and the IMAP copy works but handles a single folder per pass.
Thunderbird is the natural next home for Pegasus Mail users. Both are free and both keep mail on your own disk. It still gets updates as well. The one snag is that Thunderbird's import wizard knows about Outlook and other Thunderbird profiles, and knows nothing of the .PMM and .CNM files a Pegasus mailbox is made of.
So the mail has to be translated into Thunderbird's own MBOX format. The steps below do that in one pass, writing straight into the profile. After them come the MBOX-and-add-on route for people who want files they can archive, and the IMAP route for people who want to spend nothing.
Can Thunderbird Import Pegasus Mail?
Not on its own. Open Tools > Import in Thunderbird and the choices are another Thunderbird profile, Outlook or address book and calendar files. Pegasus Mail, David Harris's client from 1990, is not on the list and never has been. Pegasus for its part can copy mail to an IMAP server but has no MBOX or Thunderbird export.
What Thunderbird does understand is MBOX, the plain text mailbox format it uses for every folder under Local Folders. So the job is to turn each Pegasus .PMM folder and its .CNM messages into MBOX and put the result where Thunderbird looks. Univik Pegasus Converter does both halves: it reads the Pegasus files without Pegasus installed and can write directly into the Thunderbird profile, or hand you the MBOX files to import yourself.
What Are the Steps to Get Pegasus Mail into Thunderbird?
Five steps, about twenty minutes plus the copy. The client has to be closed while the converter writes into its profile, which is why installing it comes before the conversion.
Copy the Pegasus user folder
Quit Pegasus Mail. Go to C:\PMAIL\MAIL and copy the folder named after the user. It holds the whole mailbox: a .PMM file per folder, a .PMI index beside each and one .CNM file per unread message. Any Windows PC can do the rest, so the old machine only needs to hand this folder over.
Install Thunderbird and open it once
On the PC that will run Thunderbird, install it from thunderbird.net and start it once. You can skip account setup for now. That first launch creates the profile under %APPDATA%\Thunderbird\Profiles and a Local Folders account, which is where the Pegasus mail will land. Close Thunderbird before the next step so nothing is holding the profile open.
Load the Pegasus folder in Univik Pegasus Converter
Start the converter and click Open. Point it at the folder you copied. On the original PC you can instead pick a Pegasus account straight from the Open menu. The tree shows every folder with the nesting Pegasus used. Untick anything you do not want carried over, such as old newsletters or a Deleted folder.
Pick Thunderbird as the output and run it
Choose Thunderbird from the save list. The tool finds the Thunderbird profile on the PC and writes each Pegasus folder into Local Folders in Thunderbird's own MBOX format, keeping the folder tree and the message dates. If you would rather keep control, choose MBOX instead and import the files yourself, which the next section covers.
Open Thunderbird and check Local Folders
Start Thunderbird. Under Local Folders in the left pane the Pegasus folders are listed by name, subfolders inside their parents, every message with its original date and attachments. If a folder shows a count but no messages, right-click it, choose Properties and Repair Folder to rebuild its index.
How Do I Import Pegasus MBOX Files into Thunderbird?
Some people want the MBOX files in hand, as an archive or to move to a Mac or Linux PC later. In that case choose MBOX as the converter output instead of Thunderbird. You get one .mbox file per Pegasus folder, named after the folder, in a location you pick.
To bring them into Thunderbird, install the add-on first: open the menu, choose Add-ons and Themes, search for ImportExportTools NG and click Add to Thunderbird. Then right-click Local Folders, choose ImportExportTools NG > Import mbox file, pick Import directly one or more mbox files and select your files. Each one becomes a folder under Local Folders. The add-on is free and maintained. It is the standard way Thunderbird users bring in MBOX from anywhere, Gmail Takeout included.
Is the IMAP Copy to Thunderbird Worth It?
For a small mailbox, yes. Give both programs one shared IMAP account, copy each Pegasus folder into the IMAP account from inside Pegasus and wait for the upload. The client then pulls it down. Then drag the folders from the IMAP account into Local Folders if you want them stored locally. No converter, no add-on, nothing to pay.
The limits are the usual ones. It is folder by folder by hand, so a mailbox with sixty folders is a long evening. Every message goes through a server. Nested folders sometimes come back flat or renamed. And it needs Pegasus Mail to open on the old PC, which after a Windows upgrade is not guaranteed. Fine for a dozen folders, slow and fragile for a full archive.
Thunderbird users tend to be the people who chose Pegasus for the same reasons twenty years ago. They want the mail on their own disk in a format they can read without us. MBOX is that format, and writing it straight into the profile saves them the import step.
Which Route Should You Take?
Each route finishes with the mail under Local Folders. The differences are in cost, effort and what happens when Pegasus no longer opens.
| Question | Direct to profile | MBOX, then add-on | IMAP copy |
|---|---|---|---|
| Steps | Copy folder, open, pick Thunderbird, run | Copy folder, convert to MBOX, import with add-on | IMAP in both apps, copy each folder |
| Add-on needed | No | Yes, ImportExportTools NG | No |
| Keeps nested folders | Yes | Yes | Often flattened |
| Pegasus must still open | No | No | Yes |
| Thunderbird must be closed | Yes, during the run | No, only during import | No |
| Good for | One PC, quickest path | Keeping MBOX files as an archive too | A handful of folders |
| Cost | Trial, then paid | Trial, then paid | Free |
Univik Pegasus Converter turns PMM, CNM and PMI files into a Thunderbird profile or into MBOX and keeps the folder tree either way. It also produces PST, EML and PDF. It can upload to Office 365, Gmail or any IMAP account.
Windows 7 to 11 · no Pegasus needed · the trial converts a sample from each folder
The short version
- Thunderbird's Import wizard does not read Pegasus files, so the mail has to be converted to MBOX.
- Quickest: copy the Pegasus user folder, open it in Univik Pegasus Converter, pick Thunderbird, run with the client closed.
- Archive-friendly: pick MBOX instead, then import with ImportExportTools NG by right-clicking Local Folders.
- The mail lands in %APPDATA%\Thunderbird\Profiles, under Mail, Local Folders, one MBOX per folder.
- The IMAP route costs nothing but runs one folder at a time and only while Pegasus still opens.
Convert Pegasus Mail for Thunderbird, free trial
Migration Questions
About This GuideLeena Taylor Paul prepared this guide for Univik, which has built Windows email tools since 2013. The Thunderbird import options, the ImportExportTools NG menu path and the profile location were checked on the current Thunderbird release in September 2026, and the direct-to-profile output was tested from a Pegasus install that had both IMAP and POP3 accounts set up. 22 September 2026. Folder missing after the move? Support is a click away below.
Sources: University of Michigan ITS: import MBOX into Thunderbird · Mozilla Thunderbird support · Univik Pegasus Converter