setting Eudora up for IMAP

[John B. Smith]

I've been struggling to get my ISP to accept my outgoing Eudora email.
I've been experimenting with Thunderbird. I found I can create a new
'Profile' (Run-thunderbird.exe-p). Trying to enter my email address in
the new profile I find out Thundbird won't accept just any email I
type in, it has to search for and verify the address, afterwhich it
magically queries that server and zaps back the required settings into
my new Profile. I can then check the settings sent back and see what
works. The setting I'm interested in is IMAP. That's what's in a
Thunderbird Profile that works to both send and receive (in
Thunderbird).
Now Eudora, of course, lacks the capability to go out and query my
email's correct settings, I have to enter them manually.
Tools-Options-IncomingMail POP or IMAP. Choosing IMAP opens a window
asking for an IMAP mailbox location prefix. I don't know what that is.
Can anyone tell me what to put in there?