I entered user@example.org but accidentally mispelled the address. then Gmail will most likely save the email for future reference.
However, when you send your email, an email pops up in your Inbox saying "Mail Delivery Notification (Failure)" of the incorrect email address you used. You now notice your mistake, and go and compose a new email with the correct email into your "To" box, but you notice that the incorrect email address comes up as a suggestion! How to get rid of it?
Best Answer
Go to Contacts and in the left-side menu click on Other contacts (last item from the list). From there, either scroll until you find the respective contact or search for it in the search field. When you find it, click on the three vertical dots in the far-right side and then click on delete.