If you want to archive from your IMAP accounts in Mail Archiver you need the passwords of those accounts. When you have trouble finding the passwords I will show you how to fish them out of the Keychain Access app.

You have added an IMAP account in Mail Archiver.

IMAP account in Mail Archiver

But you got the wonderfully expressive error message “Authentication failed”.

Error "Authentication failed"

Of course, you first need to check the username and the servername. When those are correct you can check if the same account works fine in Mail.

Same account in Mail

The number of dots, by the way, is not an indication of the password length. Otherwise, it would be easier to guess the password.

Open the Keychain Access app:Keychain in Alfred

Now we see the main interface of the app:

On the right side you have a sidebar and the rest of the app shows all the different keychain items. In the sidebar make sure that you have selected “login” or “iCloud” as Keychain. Select “All items” or “Passwords” as Category.

Type in “com.apple.account.IMAP” into the search field. Now we got a more manageable number of keychain items as result:

Step through the items until you find the one that you need:

Found the correct account

Double-click on the item to get the password window. Click on the “Show password” checkbox, enter your login password and you will finally see the password for the IMAP account:

Show the password

That's it. Now you can copy the password to Mail Archiver.