Click Write to open an new message. Click Contacts. Click the Collected Addresses dropdown. If the address you want to delete were in your Address Book, we wouldn't be having this discussion. Check Collected Addresses, where the offending address may be located. Most likely the offending address has been hidden in Individuals. Right click/properties to see which address goes with which entry. (There will most likely be multiple entries, including the one(s) you want to keep and the one(s) you want to delete.) Once you have located the offending address, right click/delete. (Other options for deleting may also work once you have located the address.)