mail function will invoke the
sendmail binary to send a message. So there is not a need for the postfix daemon to be running.
Sendmail will place the email in the maildrop queue. When postfix starts running, the
pickup daemon will pick the message and send it. When postfix isn't running, emails stay in maildrop queue
forever until they are picked up when postfix runs again.
Resources: Postfix Architecture Overview
Local submissions are received with the Postfix sendmail(1) compatibility command, and are queued in the maildrop queue by the privileged postdrop(1) command. This arrangement even works while the Postfix mail system is not running. The local pickup(8) server picks up local submissions, enforces some sanity checks to protect Postfix, and gives the sender, recipients and message content to the cleanup(8) server.
If so, is there a way to easily nuke any queued messages
Yes. Because the email is stored in maildrop queue, you can run
postsuper -d ALL maildrop to nuke all of them. See man 1 postsuper.
# postsuper -d ALL maildrop
postsuper: Deleted: 1 message