I’m trying to clean out some of the 18,000 email message that I’ve saved. Many of those messages are should be deleted but it’s difficult to wade through them. The best way I’ve found is to search for those messages I know need to be deleted, mostly from mailing lists. The gmail syntax is: in:inbox from:search term
When filling in the email address while creating a new email message, saved addresses that were used in the past will often be filled in automatically as you type, even if the address was mistyped originally. To remove the bad address, in Mail go to Window>Previous Recipients. Find the bad address and remove it.