New Macbook won't stop adding an old Microsoft Exchange account

I seem to have solved it myself. I tried unsyncing iCloud keychain and rebooted, and the account didn't get added all day. Resynced, and it popped up right away. So it was just a matter of looking more carefully in that keychain — I'm not sure which thing I deleted ultimately did it, but there were a couple things in there that seemed like they may have related to my old login details for the work computer I was using while employed there. I rebooted, turned syncing the keychain off and on again, and the email account didn't pop up. I may be in the clear.

Well, it, uh, happened again. Seems to happen at roughly the same time each day, somewhere between 3:00pm and 3:30pm my time. I took a screenshot of the popup I get when I go to delete it from Internet Accounts:

That last bit I edited out is a 2013 Macbook Pro I was briefly using prior to this new one, and which will no longer get through the whole startup process, so there's no way for me to play with any settings on it.

Those passwords in my keychain, which I deleted yesterday, were the only reference to my old company I can find anywhere in my settings on either this computer, or my phone. My other devices are the 2013 Macbook Pro that's been turned off this whole time and which I can't access, and the 2009 Macbook Pro that's also been off this whole time. I just booted it up and confirmed there's no mention of my old company in Internet Accounts, Keychain, etc., and signed out of iCloud before shutting it down again.

When it happens again tomorrow, before removing the account, I'll check the keychain again and see if those passwords somehow popped back in.

I visited https://appleid.apple.com/ and ran through all my info to make sure the email account doesn't appear anywhere, and just for kicks I also went to https://www.icloud.com/ and confirmed the same. I'm at a complete loss at this point.

I can't find it associated with my iCloud account in any way. On my iPhone, when I go into Settings > Apple ID > Name, Phone Numbers, Email, it's not listed and never has been. I don't have my Keychain synced to iCloud, and even if I did, as I said, I'm pretty sure I never used a personal device to sign into the account (and I definitely did not do anything related to iCloud or anything personal on my work device from that company). Do you think there's anywhere else I should be checking in iCloud settings that may list it somewhere?

This started happening when all I'd done was sign in to the computer with my Apple ID. That first day, I'd also downloaded Chrome and Spotify, neither of which have any info about this work email. After a few days I transferred lots of files from my old computer to it; I'm not sure if that's what you mean. But the problem had started before that point.

I appreciate you puzzling this out!

It just happened again, and I noticed two things: one, in the Internet Accounts panel, the iCloud account had Keychain checked. I had this unchecked on the settings on my phone and assumed that setting would carry over to any device signed into iCloud, but apparently not? I also saw that, when I deleted the Exchange account, the confirmation popup mentioned Keychain again. So I hit okay, and then went into the keychain and, yep, I must have logged into the work email from a personal device at some point, because there was an entry for that email in there. I've deleted it. I'll update tomorrow with whether that stopped it!

Here's the "JavaScript embedding" link from Pastebin of my EtreCheck report. Haven't tried Safe Mode yet, as the Exchange account got added the moment I turned the computer on this morning, and I figure I missed my chance for the day. I'll reboot it in Safe Mode tonight and see what happens tomorrow.

<script src="https://pastebin.com/embed_js/7E2iyFgi"></script>

Sorry, missed part of your instructions; looks like it should just be:

https://pastebin.com/embed_js/7E2iyFgi

Ah, okay — sorry, entirely new to Pastebin. Thanks for helping out; I appreciate it! Still unwanted-account-free more than 24 hours later, by the way, so I'm calling this solved!