Google saves contacts from people you have received messages from as well as those you have contacted, so when people change their email address you may end up with their old address "stuck" in your Contacts. Because addresses are completed automatically from your Contacts, this can result in inadvertently sending to an obsolete address. Here's how to remove a bad address from your Contacts:
1) Click on "Contacts" (upper left)
2) Contacts are grouped on the left. Make sure "All Contacts" is highlighted.
3) Check the boxes for the contacts you need to remove, then click "More" and "Delete Contact." Use the search bar to search for specific addresses or domain names if necessary.