Mail.app keeps changing my SMTP settings?
You could check if macos is trying to manage your connection settings automatically. That is controlled from inside the preferences for the Mail.app, under Accounts, and under Server Connections, at the bottom of the page. If the checkbox "Automatically manage connection settings" is checked, then macos will try to manage these settings for you. There's more information about what it does and how to disable it here: https://support.apple.com/en-us/HT204208 

If that does not help, you could also remove that account completely. However this is a more drastic operation. In System Preferences (not in the Mail.app), there is a preference pane for Internet Accounts. Try removing your email account and then readding it. There's a tutorial here that explains how: http://osxdaily.com/2016/04/04/delete-email-account-from-mac/