Auto fill

The “old” one is not listed in my mail settings and so I can’t delete it.

Check your own Contacts, may be that “old” one is still there.

Check -> go to -> Settings -> Safari -> AutoFill : 

• Use contact info -> turn it Off.
• Clear All -> Clear AutoFill Data

Then restart the iPhone.

My new email address is not replacing an old one that keep appearing as an auto-fill suggestion.

Try this -> go to Settings -> Mail -> Accounts -> scroll & tap that old email address -> scroll down & tap Delete/ Remove account.