If you’ve been using Thunderbird for a while it has probably built up a good-sized address book for you. Cleaning it out every now and then is a good practice to avoid making addressing mistakes.
Auto-fill of known addresses is a handy feature, but if you’ve got lots of old addresses hanging around it can cause headaches and mistakes if you’re not paying attention. For instance, usernames at Hampshire should always be unique, but email aliases, which are composed from the person’s first initial and last name, may be re-used. It’s also easier to type the first few letters of someone’s name and home in on the right correspondent if your address book is in good shape.
When you open up your address book in Thunderbird, you will notice there are at least 2: Collected Addresses and Personal Address Book. There may be more, including Mac OS X Address Book, depending on what type of system you’re working on and whether you’ve imported any address books in the past.
Open up each address book, and just scan the list quickly, looking for people who have moved on & with whom you no longer correspond. You can select a name and press the Delete key (Mac) or Backspace key (PC) to remove it. This can be a tedious project, but you can break it into different sessions if need be.