If you are receiving this error, then try the following workarounds:

Compact your folders
Even though you have emptied your Trash folder, those deleted files still remain in the folder, on the HD, until the Mail is compacted

If no joy, then with the program closed:
Open Windows Explorer
Navigate to your Mail folder, and delete all files with an MSF extension
Don't delete the files without the MSF extension, as they contain your mail. See: Profiles Folder Location

Still having the problem?

Use your ISP Webmail Interface, delete any spam messages, or those with no message body.
Delete anything that looks suspicious to you.

Next:

Close the application:
Open the profile's Mail Account folder ...\MAIL\pop.ISP.net
Rename the Inbox file with NO MSF extension, to OldInbox, and delete the Inbox.msf file.
When the Mail window is reopened you should see a pristine Inbox folder and a folder named Oldinbox.

If you are unsuccessful with the above workarounds, then create a new Profile, as outlined in this guideline:
Creating a New Profile

If you continue to have problems, then post your questions on the:

WindowsBBS Netscape and Mozilla Forum.

I'm the Moderator on the Forum, and along with the other informed members,
will be happy to provide you with any further assistance you might require.

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Ramona