Problem: When you open Eudora you get an error message that states:

Remote Instance
An instance of Eudora may be running on a remote computer. Accessing a mailbox from two instances of Eudora may lead to data corruption. Please indicate how you wish to proceed.

Exit this instance
Terminate remote instance
Continue (I'll accept the consequences)

This error occurs when Eudora detects a file named owner.lok in the same folder as the mailboxes.
When Eudora opens, it first checks for the file owner.lok and if it doesn't exist it creates it. The presence of this file is supposed to indicate that the mailboxes are in use by another program, and prevent opening the same mailboxes twice. When Eudora closes, it is supposed to delete the owner.lok file. Sometimes this file doesn't get deleted, and this can cause the "Remote Instance" error message.

ISSUE:

When launching Eudora, a dialog opens informing that there is a remote instance of Eudora running, and gives the option to terminate the remote instance or quit.

SOLUTION:

Eudora is not designed to allow multiple users to access its files simultaneously; it has no mechanisms to protect files from being corrupted when accessed simultaneously by multiple users.

If you are not trying to open Eudora multiple times to access the same files, then you may be getting this error after a crash. When Eudora is launched, it creates an Owner.lok file, and gives you the warning about a remote instance if that file is found already existing in the Eudora directory. Crashing out of Eudora doesn't give it the opportunity to clean up after itself and sometimes leaves a corrupted owner.lok file. Delete this file and you should be able to open Eudora again without the remote instance dialog box.

If you're using Eudora 5.1 or higher, reboot your system prior to deleting the owner.lok file.

If there is never the potential for multiple people accessing Eudora simultaneously and you would like the owner.lok creation and check to never happen, copy and paste the following text into a new message in Eudora.

<x-eudora-option:OwnerLok=0>

The text you paste will show up in blue as a URL. Hold down the ALT key and click on it - a window will appear asking you to click OK. Click OK. That will do it.

WARNING: If you disable the owner.lok creation you will not get any warning if you or another person logs into the same account where your mailboxes are stored. If two people log into your account at the same time your mailboxes will become corrupted without warning.