Have happily used PMAIL since around 1985, but due to issues beyond my control I may have to convert to Thunderbird.

I have ~30K messages, organized into lots of subject specific folders. I would (obviously) like to maintain that same organization.

Are there best practices to follow in doing this?

All advice and cautions are welcome.

(Context: have been a (mostly) Windows and (also) Linux user for since 1985, so suspect I can do most things required, if only I knew what to do. Have had enough experience to know that there can be numerous gotcha's, and doing it right the first time is infinitely easier than trying to slog through it, hence the request for guidance.)

many thanks
Randy

Pegasus Mail data storage format is unique. AFAIK, Thunderbird can't read it so there will some hoop-jumping involved. What comes to my mind is copying messages from Pegasus Mail folders to folders in Unix Mailbox Format which is one of the Pegasus Mail new folder options. That data is then in .mbx files which is a type of MBOX. There are plenty of sources on the web that talk about Thunderbird and MBOX, but not much about .mbx specifically. One post I found says that you can copy a .mbx file (without an extension) to the Thunderbird 'Local Folders" directory and Thunderbird will recognize it. This would mean creating as many Unix Mailbox Format folders as you need in Pegasus Mail, copying messages to them, then copying those .mbx files (renamed without the extension) to the Thunderbird 'Local Folders' directory. You will certainly have to do some testing.
This is that article I found: https://support.mozilla.org/en-US/questions/1273316

Hopefully someone knows of an easier way to turn the contents of a Pegasus Mail folder into an .mbx file without the manual process of copying to a newly created Unix format folder.

If it is an option, perhaps you can keep Pegasus Mail running for archive purposes and start fresh with Thunderbird.

Yes, I tested it and it works. The difference between PM's folder format and MBX is like follows:

• Each PM folder (using the PMM extension) starts with exactly 128 Bytes of administrative data containing its internal and the user friendly name without a terminating line break. Messages in folders are separated from the respective previous one by a single first 0x1A byte value on a new line immediately before the next message's main header section starts.

• MBOX folders are separating single messages by a leading FROM header line, so when messages are copied to an MBX file the 0x1A byte is replaced by a FROM header looking like this: From ???@??? Sun Aug 09 01:37:40 2026 (i.e. a dummy address followed by the creation date of this replacement and a CRLF line break).

For those being capable of coding this can be implemented as a simple conversion script.