A mail spool is a file that stores the mail header (i.e. sender's address, time of delivery, etc.) and message body of every email for a particular account. Sendmail uses mail spools; qmail does not.
On a VPS, mail spools are located in the /var/spool directory. Linux user mail spools are located at /var/spool/mail while virtual mail user mail spools are located at /var/spool/vmail/sitename.com/mail.
Email addresses in the form of username@(multiple domains) are Linux user email addresses. Email addresses in the form of email@example.com are virtual email addresses.