If you didn't reboot the system sometime the day before the email was sent, you may find a reason for the reboot in the syslog from the .system dataset. Info might have been output to the log just before the reboot.
I'm here
/mnt/storage/.system/syslog/log
Ho do I read the info you were writing about?
Here's the output of ls -lah
ls -lah
total 684
drwxr-xr-x 4 root wheel 39B Mar 24 03:01 .
drwxr-xr-x 3 root wheel 3B Mar 15 18:09 ..
-rw-r--r-- 1 root wheel 1B Mar 24 03:01 3ware_raid_alarms.today
-rw-r--r-- 1 root wheel 1B Mar 23 03:01 3ware_raid_alarms.yesterday
-rw------- 1 root wheel 35k Mar 24 21:10 auth.log
-rw------- 1 root wheel 241k Mar 24 21:11 cron
-rw------- 1 root wheel 22k Mar 22 23:00 cron.0.bz2
-rw------- 1 root wheel 21k Mar 20 14:00 cron.1.bz2
-rw------- 1 root wheel 13k Mar 24 03:01 dmesg.today
-rw------- 1 root wheel 2.5k Mar 22 03:01 dmesg.yesterday
-rw-r--r-- 1 root wheel 531B Mar 24 21:10 lpd-errs
-rw-r----- 1 root wheel 1.4k Mar 24 21:10 maillog
-rw-r----- 1 root wheel 796B Mar 24 00:00 maillog.0.bz2
-rw-r----- 1 root wheel 121B Mar 23 00:00 maillog.1.bz2
-rw-r----- 1 root wheel 560B Mar 22 00:00 maillog.2.bz2
-rw-r----- 1 root wheel 567B Mar 21 00:00 maillog.3.bz2
-rw-r----- 1 root wheel 118B Mar 20 00:00 maillog.4.bz2
-rw-r----- 1 root wheel 693B Mar 19 00:00 maillog.5.bz2
-rw-r----- 1 root wheel 562B Mar 18 00:00 maillog.6.bz2
-rw-r--r-- 1 root wheel 332k Mar 24 21:10 messages
-rw-r--r-- 1 root wheel 28k Mar 21 16:00 messages.0.bz2
-rw-r--r-- 1 root wheel 25k Mar 18 02:00 messages.1.bz2
-rw------- 1 root wheel 1.7k Mar 22 03:01 mount.today
-rw------- 1 root wheel 1.6k Mar 21 03:01 mount.yesterday
-rw-r--r-- 1 www www 625k Mar 24 21:10 nginx-access.log
-rw-r--r-- 1 www www 12k Mar 20 14:00 nginx-access.log.0.bz2
-rw-r--r-- 1 www www 0B Mar 15 18:04 nginx-error.log
-rw-r--r-- 1 root wheel 1.1k Mar 24 21:10 pbid.log
-rw------- 1 root wheel 0B Mar 16 03:01 pf.today
-rw-r----- 1 root network 531B Mar 24 21:10 ppp.log
drwxr-xr-x 2 root wheel 2B Mar 15 18:04 proftpd
drwxr-xr-x 2 root wheel 8B Mar 15 18:09 samba4
-rw------- 1 root wheel 531B Mar 24 21:10 security
-rw-r--r-- 1 root wheel 1.2k Mar 24 21:10 ups.log
-rw------- 1 root wheel 8.5k Mar 24 21:10 userlog
-rw-r--r-- 1 root wheel 394B Mar 24 21:10 utx.lastlogin
-rw-r--r-- 1 root wheel 4.7k Mar 24 21:10 utx.log
-rw-r--r-- 1 root wheel 0B Mar 15 18:04 wtmp
-rw------- 1 root wheel 531B Mar 24 21:10 xferlog