The from utility prints out the mail header lines in your mailbox file to show you who your mail is from. If username is specified, then username's mailbox is examined instead of your own.

-s sender

Only display headers for mail sent by sender.

See largefile(5) for the description of the behavior of from when encountering files greater than or equal to 2 Gbyte ( 2³¹ bytes).

/var/spool/mail/*

See attributes(5) for descriptions of the following attributes:

biff(1B), mail(1B), attributes(5), largefile(5)