Let's say, you want to send an email to
firstname.lastname@example.org but you accidentally wrote
email@example.com (spelling error), 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?