Q: Can't add new OR change existing SMTP server in Mail app

For the past three or four days Mail app has been consistently chocking on connecting to my mail servers, which are mostly Google Apps for the domain, but a few others (and all have had problems). All accounts are IMAP.

Now, as these problems started I began receiving emails from Google along the lines of "an app using an insecure method tried to access your account and was blocked". Hmmmm, so I went through the process to allow "insecure" connections to Google and the mail worked again (occasionally, but went Offline constantly).

So I dug around in the Prefs > Accounts and saw that under the SMTP settings the Authentication was set to None. So I set it to Password and filled in the info. But when I closed that and went back to Accounts list, the one I was in flashed (the one highlighted on the left, the blue select background flickered). I closed the window, got the "Save" dialog, which I did, checked mail. It worked for a bit, then the same problems returned.

I went BACK to Prefs > Accounts and the Authentication method was back to None. Hmmmmm. I repeated the process, but every time it returned to none.

I then tried to add a NEW (duplicate but diff name) SMTP server with the correct settings. I saved it, then switched the account to it, saved again...and here is where it gets really weird--when I went back to check on the account, the new SMTP server I had set up was GONE, and no server was selected for the account. <headache>

There is a checkbox on the SMTP server sheet that says "Automatically detect and maintain account settings" which is checked, but grayed out so I can't deselect it.

I'm at my wits end. Nothing about this makes sense. I tried deleting one of the accounts and starting from scratch by choosing "other" instead of the "dummy proof" options, but that didn't work. You HAVE to select "Google" apparently in order for it to work (and I suspect this is why that "Automatically maintain..." box is grayed out).

Any kind soul out there with a suggestion? I searched through many posts about mail app email problems, but found nothing that helps or resembles this problem.

I'm on the latest version of El Cap on the latest rev of MacBook Pro 15 Retina.

ps.

I did run Disk Utility to confirm no disk errors.