If your local mail spool is a Berkeley mailbox file, try
grep '^From cron ' "$MAIL"
This relies on the fact that Berkeley mbox files use a line starting with
From, the sender's envelope address, and two spaces (usually followed by the message's arrival time) as the separator line fefore each new message. (A consequence of this is that the message body cannot contain the five characters
From at beginning of line anywhere in it.) If your
cron daemon's messages contain e.g. a FQDN domain name, you'll obviously need to adapt the regex.
Berkeley mbox used to be ubiquitous, but these days, many systems use other mailbox formats, many of which are folder-based, rather than a single flat file. For maildir, a common format especially for IMAP mailboxes, try
find new cur tmp -type f -exec grep -m 1 '^From:' /dev/null | grep cron
which examines the
From: header in each message (this is distinct from the similarly labelled Berkeley separator line, which isn't a mail header at all). You'd run this in the folder you want to examine, or include the full path to the
tmp subdirectories in the directory arguments.