Q: Mac Os Mail cannot uncheck "Automatically detect and maintain account settings" on SMTP server list

Do you use iCloud Keychain?

Back up all data.

Open the iCloud preference pane and uncheck the Keychain box. You'll be prompted to delete the local iCloud keychain. Confirm—the data will remain on the servers. Test to see whether there's any change in the problem. Then re-check the box. Follow one of the procedures described in this support article to set up iCloud Keychain on an additional device. Test again.

Try disabling the account and then see if you can disable Automatically maintain..........

I have the same problem.  Tried your trick, but still can't uncheck that #$@#!## box.  Anyone have success with this?

I was having the same issue with outgoing mail (it would not recognize my SMTP server)... Auto-detect Settings was checked and greyed out so I couldn't actually change it if I wanted to.   Solution:  I disabled the email account, waited for 5 minutes so things could happen behind the scenes, then enabled it again.  This seems to have resolved the issue.

Now if I could only get it to remember my email client account password so I didn't have to enter it once a day.............................

I had this problem. I had three accounts enabled on mail, and I could uncheck the 'Automatically detect settings' box on two of them, but not the third (my main account). I fixed it this morning after two weeks (happened when I upgraded to El Capitain). I disabled the account through 'internet accounts' and unchecked every box imaginable so the account was completely gone from internet accounts and mail accounts. I tried reconfiguring from there initially but it didn't work. The thing that fixed it was to sign out of my apple id on the computer, sign in again on a different apple id (my husband's) and then add my main email address back to the mail accounts. The email account that wasn't sending was also my apple id. Mail is now sending from my main account, but I haven't tried signing in to the computer as 'myself' again with my apple id. I'm not very technically gifted so can't say why this is happening, but this seems to be where the problem is.

I have the same problem now. Did you ever find a solution?

follow apple article : Automatically detect and maintain account settings in Mail on your Mac - Apple Support

totally unhelpful... the article does not explain or answer this question.