Sometimes, when I log into a box and 'su' to root, I get a cute little message saying I have mail (thank GOD it's not AOL).
Is this even actual "mail" in the same sense as email? Or is it just my system telling me something?
EDIT: In relation to this question, would I be able to send myself mail using the sendmail program like so: email@localhost?
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It's likely to be in the spool file: /var/mail/$USER or /var/spool/mail/$USER are the most common locations on Linux and BSD.
(Other locations are possible – check if $MAIL is set – but by default, the system only informs you about /var(/spool)/mail.)
Usually the spool file is in a very simple mbox format, so you can open it in a text editor or pager.
For a slightly more convenient way, most distributions come with a program called mail (or Mail, mailx). You can try mutt or alpine; you can even configure it to be sent to an outside mailbox. (See "is this real mail?" below.)
Most often the messages contain output of cron jobs, or a system security report by logwatch, or similar junk. Read it and find out.
Depends greatly on the contents of each message.
You should at least scan the subject headers – often people ignore the mail for months never realizing that their daily cron jobs fail.
Yes to both – it's generated by your system telling you something, but it's also actual email and can be handled as such.
You can (and should) configure your mail software – the "MTA" aka /usr/sbin/sendmail – to forward the messages to your personal mail address. The exact instructions vary depending on which MTA (if any) you have installed, whether this is a personal computer or a server, whether you have your own domain or use a @gmail.com, and so on.
Note that /usr/sbin/sendmail nowadays is a shared API and doesn't necessarily mean the original Sendmail MTA. In fact, you shouldn't use Sendmail, but something more modern like OpenSMTPD, Postfix, or Exim4. All of them provide the same /usr/sbin/sendmail tool, but they're easier to configure, more secure, and just as powerful.
The easiest way for me was to run the following in a terminal:
Kompcat /var/spool/mail/root
For anyone wondering how to read these messages one by one, you can just use 'mail'
$ mail
Then type a message number from the list which you want to read.
To goto the next message you can type 'next', another useful command is 'delete'..
To see more about how the mail program works, see the output of man mail.
Just to offer some clarification, it's been the tradition for a long time for UNIX boxes to run a "locally configured" mailer daemon that doesn't route messages through the Internet, but only copies messages to other users spool directories (as @John T mentioned). It is real SMTP-compliant email, it's just not routed over the Internet because it doesn't need to be.
So, if you fire up an email client (like alpine or mutt) you can send mail to other users on the system by addressing to user@hostname, or in many cases, just specifying the user. You also likely have a command line utility called mailx that can be used to send a quick email using your shell.
Now, if you want to send messages to users on systems other than the one you are logged in to, that's where the real fun begins. You'll need to configure your mailer daemon to recognize and hand off external email to a transfer agent, configure the transfer agent, and do a whole bunch of stuff I don't yet understand myself.
Where is this mail?
Very likely stored on your machine, in the mailbox folder for your root user (where THAT is depends on what kind of mail server you have installed). You can probably access it quite quickly through alpine.
What does it contain?
Probably just some system report type stuff. "So and so tried to login 10 times with an incorrect password".. "I wish someone would run updates on me", "driver such-and-such is failing constantly", "The NSA bot has boosted performance by 1%".. etc..
Who/What sent it?
Services on your server, most likely.
How important is it?
Depends on how important your server is.
This mail is typically located in /var/spool/mail/root when the (usual) default sendmail daemon is configured. It contains a RAW e-mail message that can be read by the mail utility. Who sent it can be found by running the mail utility and paging through the messages or reading the RAW headers.
The importance of the message usually depends on the previous factor, who sent it :)
Many system utilities will use the mail subsystem as a means of reporting and logging. Log watcher utilities and other services may send an e-mail to the local root user as a reminder about a specific event, such as low disk space or hardware errors. The mailer daemon will also alert you if it failed to send an e-mail outbound.
less +G /var/spool/mail/root
shows the latest mails, at the end if the file.
Directly showing the mail file is a good idea, but using cat can easily go wrong. You do not want all mails scroll by, if there are many messages, or very long messages in the file. I just saw one with 150MB, two million lines.
If you are not root:
sudo less +G /var/spool/mail/root
The +G option is used to show the newest message first; It is not at the top of the file, but on the end.
It is an option of less, making it show the last page of the file after starting. That show the last message (newest), if it fits to one page, or the end of that otherwise.
The file name may be /var/spool/mail/mail instead of /var/spool/mail/root
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