E-mail has evolved from an alternative to snail mail to an essential communication channel for business operations.
POP3 & Webmail may be great for personal use, but their lack for certain essential features have led corporations to choose IMAP e-mail systems such as Microsoft Exchange or Lotus Notes instead.
IMAP Servers store e-mail in a central server so that all e-mail are backed-up & available all the time. IMAP Servers have always been expensive due to it's huge appetite for storage space. Even though prices of storage has dropped a lot over the years, the price of Microsoft Exchange, Lotus Notes & other "Enterprise" IMAP e-mail systems have not.
Open Source IMAP Servers provide a lower-cost alternative to proprietary products. GSPL has the answer for you.