Mail account using the wrong server

Here's a weird one...

Out of the blue I can't send email from my Google account in Mail. Each time I try it gives me an error message. (See the uploaded screenshot below.)

Why is iCloud getting involved when I'm using my Google account? That's never happened before.

If I select the Google server instead (the big green arrow), and click the "Try With Selected Server" button it works like it should.

I did recently add a second Google account to Mail that I use for an organization I attend. I use it rarely and have no problem sending email from that one. Other than that, I've not messed with Mail's accounts.

I have rebooted multiple times. I've temporarily removed the firewall to test. I've deleted the Google account from System Settings > Internet Accounts and added it again via Mail. Nothing's made a change. I've also tested the same Google account via my iPhone and via Gmail's webpage. They both worked fine, so it's only on my Mac.

One theory: Way back in the day when I created my iCloud account I used this Google email as my login. However, considering that this account works on my iPhone, I doubt it is this. Thought I'd mention it though.

Any ideas? Thanks in advance!

I'm on a Mac Studio (2023) running Tahoe 26.0.1.