[Mozilla/Netscape]
In Mozilla and Netscape Mail, there's a neat shortcut you can use to discover the exact location of a particular mail folder:
For example, the path to my Inbox in Mozilla Mail is:
mailbox:/C|/Documents and Settings/rose/Application Data/Mozilla/ Profiles/Rose/l3xn4x9v.slt/ Mail/pop.east.cox.net/Inbox
That's not in the usual form you see pathnames, but it's easy enough to derive the regular pathname -- just strip off the mailbox:/, replace the C| with C: , and make all the slashes into sloshes (backslashes)..
Here's a quick way to open a mail folder on your computer:
C:\Documents and Settings\Rose\Application Data\Mozilla\Profiles\Rose\l3xn4x9v.slt\Mail\pop.east.cox.net\
If you open the mail folder you'll find two files for every mail folder. For example, you'll find an Index file and an Index.msf file; a Trash and a Trash.msf; a Sent and a Sent.msf; and so on. The .msf files are indexes; your mail is stored in the other files, those without file extensions. If you ever do a manual backup, you only really need to copy the extension-less files because the .msf files get re-built automatically, although you may find it easier to copy the entire contents of your mail folder anyway. If you have any nested mail folders (for example, Work\Read Later and Work\Done) you'll find a sub-folder with an .sbd extension with the mail files contained within that. In this example, there'd be a sub-folder called Work.sbd containing four files: Read Later, Read Later.msf, Done and Done.msf.
[Outlook Express]
In Outlook Express, right-click a folder and choose Properties from the pop-up menu to see the storage location. In the Properties dialog the folder location is displayed under This Folder Is Stored In The Following File. Chances are the folder name will be truncated, due to shoddy design on Microsoft's part. And, because it's greyed out, you may think there's no way of displaying the whole thing. But you can: Just click anywhere in the folder path and use the right-arrow key to scroll through it. If you want to grab the whole path, click and drag all the way across the folder path to highlight it, press Ctrl+V to copy it to the clipboard, and then paste it into a document.
Outlook Express stores my Inbox in an even more outlandishly out-of-the-way place than Mozilla:
C:\Documents and Settings\rose\Application Data\Identities\{742DC712-BE9D-4EF7-BA65-C0041FC3121A}\Microsoft\Outlook Express\Inbox.dbx
To open the folder:
C:\Documents and Settings\rose\Application Data\Identities\{742DC712-BE9D-4EF7-BA65-C0041FC3121A}\Microsoft\Outlook Express\
[Outlook]
Outlook stores all your mail in a single .pst file (if you're using POP3 email and not Exchange Server), instead of using individual files for each mail folder. In fact, Outlook sticks everything into that .pst file: Mail, contacts, calendar entries, and so on. To locate the .pst file, by default called Outlook.pst:
[Eudora]
If you install Eudora to the default location, you'll find the mail files in the C:\Program Files\Qualcomm\Eudora folder. Eudora creates two files for each mail folder, the mail store has the extension .mbx and the index has the extension .toc.
[Pegasus]
Pegasus creates a folder for each user within C:\Program Files\PMAIL\MAIL (provided you installed the program to the default location). To check the location for the current user:
Pegasus has a particularly intricate system of storing messages, mail, folders and all the other stuff it keeps (if you want to sort out what all those files are in your PMAIL\MAIL folder, check out this item from the Pegasus Knowledgebase. If you ever want to back up your Pegasus mail, grab the whole folder.