If despite not changing any settings, you start not receiving any emails, OR if the people trying to send you emails are receiving this message:
Hi. This is the qmail-send program at lin.kalfaoglu.net.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
email@example.com: Mail quota exceeded.
That means that you have too much email accumulated at the server, and therefore you have exceeded your email quota.
This usually happens because of your email program's settings. Go into its settings and locate the "Leave a copy on the server" setting. This should be DISABLED.
However if you do not use an email client but use webmail to read/write your emails, that means its Trash needs to be cleaned or Expunged. You can do that in its settings.