How Received Email Is Processed
The mail server processes incoming email addressed to your account by checking for directives in files in your home directory. If no directives are found, then it will use the destination of last resort.
Processing of Forward Files
First, the mail server looks for and honours the contents of the file named .forward in your home directory. Directives in this .forward file tend to be relatively simple, such as as asking the mail server to forward your mail to some other account (please see How To Forward Email). If this file is found, the search for the next file, .procmailrc, is not undertaken and the destination of last resort does not to be used.
Processing of Procmailrc Files
If no .forward file was found, then the mail server looks for and honours the contents of the file named .procmailrc in your home directory. Directives in this .procmailrc file might be as simple as asking the mail server to forward your mail to some other account but can be as complicated as filtering for spam (please see How To Filter Out Spam). If this file is found, the destination of last resort is not used.
Destination of Last Resort
If no .forward file nor .procmailrc file was found in your home directory, then the mail server appends the incoming email to your mbox-formatted spool file: /var/mail/username. This is the so-called destination of last resort.
Interaction with IMAP and Webmail
If you want to use IMAP (IMAPS, actually) or Webmail, then you should have a .forward file or a .procmailrc file that causes the mail server to redirect your mail into a folder called Maildir in your home directory. The IMAP server expects to find your new mail in your Maildir. Please see IMAPS for more information.
Interaction with Disk Quotas
Mbox spool files have a quota of 100MB.
Maildir folders in your home directory are limited to your quota on the file server volume where your home directory resides or the size of the volume, whichever is filled first.
Once you reach your quota or the disk is full, email will queue for up to four days before being bounced back to the originator.
Please ensure that you remain below your quota to ensure that email is delivered to you.