When entering an email address, Zimbra tries to be helpful by offering a selection of the most recent or frequently used email addresses that might match your entered letters.

Perhaps you mis-typed it at some point and it’s an incorrect email address, or it’s an address you don’t want to use again.

There is a “Forget” link at the end of the suggested email address pop-up. Click it to tell Zimbra to remove that email address from your frequently used emails.