I'm prompted on every reboot for a password for a stale iCloud account

I have had an iCloud account associated with a Gmail ID [email protected]. I then switched to another ID [email protected], so I removed [email protected] from my iCloud account and entered [email protected]. This was an year or so back.

A few days back, I found an entry in the Keychain for [email protected], so I deleted it. After that, I'm prompted one every reboot to enter the password for [email protected]:

I cancel it, but it comes up after every reboot. I tried entering my iCloud password, but it doesn't work.

How do I get this prompt to go away and never come back?

The application icon suggests the password is being requested by Messages.app.

Messages.app

Launch Messages.app and check the accounts listed in the preferences do not include or mention your stale @gmail.com account.

Internet Accounts

Look for legacy accounts in your Internet Accounts within System Preferences.app:

Preferences

Check your preferences for mentions of the old address using the Terminal.app command:

grep -r old@gmail\.com ~/Library/Preferences/*

If this matches a file, modify the following command to examine the file's contents:

defaults read ~/Library/Preferences/MobileMeAccounts.plist > ~/Desktop/output.txt

In the example above, a new file called output.txt will appear on your desktop. This file can be opened with TextEdit.app.

This file may hint as to which application or tool has your old account listed.

If you find a matching preference, consider deleting or renaming it. The preferences will be lost but they should never contain critical information.

Remember to log out or restart after altering the preferences file.