I'm renting server space from someone and, upon logging in my control panel after quite sometime, noticed an abnormal spike (~50MB) in the disk usage. Upon investigating, I found a lot of core.* files scattered around my public_html directory. Each one is more than 5MB in size but no more than 6MB. The * part is all numbers (in programming regex, that should be core\.\d+).

I downloaded one and checked the contents. There was a lot of balderdash characters (NUL mostly, but also a scattering of ETB, ETX, STX) but there's this block of readable text which says:

This text is part of the internal format of your mail folder, and is not
a real message.  It is created automatically by the mail system software.
If deleted, important folder data will be lost, and it will be re-created
with the data reset to initial values.

Pretty self-explanatory. A few blocks above the text are some more readable messages that look like logs but is sandwiched in between non printable characters. I've extracted some below.

Scan not valid for mh mailboxes
Bogus character 0x%x in news state
Can't rewrite news state %.80s
Error closing backup news state %.80s
No state for newsgroup %.80s found

Now, a few concerns: Am I under attack? The messages seem to be about my webmail but I don't use my personal webmail that much---only for a vanity email address and an inbox for an outdated comments system. However, lately, I seem to notice a spike in the spam for my vanity mail. (Note: the comments system is covered by a captcha but every now and then some get through. My vanity email has a spam filter but it isn't as good as I'd like).

Next, if this is a feature, can I turn it off? Is it advisable to? I've only 150MB so you see why I'm fretting over a 50MB spike.

Some final details: my only server-side scripts are in PHP. The directory which accumulated the most number of these core files is the one containing the Wordpress-managed subdomain of my site. I manage my server through CPanel. Lastly, I decided to delete this files and after some checking nothing seems amiss in my websites nor in my mail. They are indeed the ones responsible for the ~50MB spike as my disk space usage is back to expected.