A while back I wrote a post about Sparrow's autocomplete function and how to purge the app's cache so that addresses you sent an email to one time long ago no longer show in the autocomplete results.
When you send messages, Mail stores your recipients’ email addresses in the Previous Recipients list. When you want to send new messages to those email addresses, just type a few letters in an address field, and Mail automatically fills in the rest.
I've accumulated a lot of email addresses that I've used only once or twice since installing Lion and it has made the autocomplete cache (and thus, the suggestions it offers when addressing an email) an absolute nightmare. Luckily, I found a way to purge the autocomplete cache, minus the addresses already listed in my Address Book. Here's how to do it:
Launch Apple Mail, go to the Window menu and select Previous Recipients.
In the resulting window, you'll see a list of email addresses and the last time you sent an email to that address.
You can select individual entries by Command+Clicking on each entry you want to select or you can press Command+A to select all.
To remove the selected entries, click Remove From List. If you want to add a particular entry to your Address Book, select those entries you want to keep and choose Add to Address Book.