If you like the flexibility of writing and sending to the queue a couple of letters, then connecting to the Internet to send them all out in one burst, your sent mailbox will have to be local. In this case, sendmail silently queues up all your mail as you write it, and shoots it off to your ISP the moment it detects an internet connection (and long before you'd have time to fire up your email program and hit the 'send' key manually). And each message sent to the mail queue is simultaneously copied to your sent mail box. If you have a broadband connection, there's no real need to store your sent mail locally, and you can use the second configuration below.
set copy=yes #keep copies of outgoing mail. set record="/home/randymon/Mail/sent-mail " #This is for locally stored mail set record="=Sent Items" #This is for the `Sent Items' folder on an IMAP account. # - note the equals sign, which makes it clear # this a subdirectory of the inbox.
Lastly, an advanced trick: you can specify which folder to use on a per-message basis using send-hooks (see section 5.10).