I'am agree with Phil..you can use Pegasus Mail to search very easy [*-)]

I use Copernic Desktop Search for many files on my PC.
I also run Mercury on this PC, and would like to be able to search for text inside .PMM files.

I have just tried Agent Ransack, which works after a fashion. (Slow with large files).
Does anyone have any other suggestion?

[Update: I've discovered I can add a custom filetype to Copernic. It is now chugging along trying to index the files (set to whole words only).]

I'm an Agent Ransack fan and consider it one of the best utilities for what you are trying to do.  The difficulty is that you are searching .PMM files.  I don't know what text you are searching for but if it is header content and you can utilize the Pegasus Mail header search function then that might be the best way to go.

No, I'm searching for 'body' content. And I'm using Thunderbird via IMAP, so I don't think I can use any email client search since it doesn't have the email contents.

After Copernic's indexing finished, I now find that I can search for a word and get a FOLxxxxx.PMM file name quickly; then I wait about a minute for the content to display.
Looks useful enough, as I get the raw email showing with From, To, Sent, Subject and all. Then I can go locate the email if I need to look at attachments, or reply.

Why don't you use Pegasus Mail just for its ability to search in .pmm files ?

HTH