is it safe to delete /var/spool/mail/root

Solution 1:

A more secure way to delete the mails is to use a mail client and delete all the messages.

If you are in debian/ubuntu, you can easily install "bsd-mailx".

with mailx you can delete the messages as follow (as root):

# bsd-mailx
...
& d *
& q

The comman d * deletes all messages and q saves the mailbox and quits mailx. You can delete individual messages, too. See manual of mailx.

There are many other command line mail clients, which could be used.

Solution 2:

Yes, as others have already said, they should be safe to delete, and yes, the best way is with a mail client.

But... Take a step back and ask yourself why they are there in the first place.

Is there anything important in there that you should know about?

Whatever sent those emails is still sending emails. For each one, configure it to only send important emails (errors and warnings?), and to an inbox that's not treated as a black hole. Then fix whatever's causing them to be sent.