Removing unwanted/expired email addresses from email suggestions list

How:

1. Log into your gmail (or gmail powered) account.
2. Go to google.com/contacts. You will see a list of email accounts.
3. Delete the email from here you wouldn't want to see email suggestions.
4. Done.

Bonus:

By Default Google saves each email account you send mail to. If you wish to change this setting

1. Log into your gmail or Google powered mail account.
2. Click the gear icon on right top and select 'settings' from drop down menu.
3. Select "General” tab if it's not selected already.
4. Now search for "Create contacts for auto-complete” setting and change it's value to "I'll add contacts myself”.