If memory serves, the sendmail binary creates a randomly named data file and a 
corresponding lock file inside the outgoing mail spool directory, writes the message into the 
main file, then deletes the lock file.

Meanwhile, the sendmail daemon (running as root) watches that queue directory and when it 
sees a new file, it waits for the corresponding lock file to disappear (if it even still exists at 
that point).  Then it reads the file, relays its contents, and deletes the file from disk.

That said, I've never done any coding under the hood of sendmail—the closest I've ever come 
is writing sendmail.cf rules—so I'm not certain about that.