Eudora

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Entire "Eudora" folder and its contents

POP mailboxes:
C:\Documents and Settings\[username]\Application Data\Qualcomm\Eudora
C:\Users\[username]\AppData\Roaming\Qualcomm\Eudora

IMAP mailboxes:
C:\Program Files\Qualcomm\Eudora\Imap\Dominant\
C:\Users\[username]\AppData\Roaming\Qualcomm\Eudora\Imap\Dominant

Mac Mail

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Entire “Mail” folder and its contents

v.1: ~/Library/Mail/Mailboxes/inbox.mbox/mbox

v.2+: 
~/Library/Mail/Mailboxes/inbox.mbox/Messages/nnnn.emlx
~/Library/Mail/Mailboxes/IMAP-[email_account]/Mail/inbox.imapmbox/Messages/nnnn.emlx
~/Library/Mail/Mailboxes/POP-[email_account]/Mail/inbox.popmbox/Messages/nnnn.emlx

archived email:  ~/Downloads/archived-mail/inboxold.mbox/mbox

Outlook Mac

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.olm export file(s)

To export:

1. Open Outlook for Mac.

2. Select File>Export (v2011) or Tools>Export (v2015).

3. Choose export options.

4. Choose a location to save the export file.

5. Outside Outlook, navigate to folder where you saved the .olm file. Copy it.

Outlook Windows

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Entire Outlook data folder and its contents

In XP: c:\Documents and Settings\[your login name]\Local Settings\Application Data\Microsoft\Outlook

In Vista/Windows 7: c:\Users\[your login name]\AppData\Local\Microsoft\Outlook

Thunderbird

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Entire mail server folder containing the INBOX mailbox file, and its sub-folders

In Windows:

c:\Documents and Settings\<user name>\Application Data\Thunderbird\Profiles\<Profilename>\<Mail | ImapMail>\<mail server name>\
c:\Users\<user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\<Mail | ImapMail>\<mail server name>\

In Mac OS X: (~ represents user's home folder):
~/Library/Thunderbird/Profiles/<Profile name>/<Mail | ImapMail>/<mail server name>/
~/Library/Application Support/Thunderbird/Profiles/<Profile name>/<Mail | ImapMail>/<mailserver name>/