Choosing the most appropriate backup option is always a trade-off. You have to juggle the cost of downtime to a business, the price and availability of backup media and hardware, the value of user data (in our case, users' e-mails), and the staffing costs to manage the backup operations.
For our small office e-mail server, we are going to present a simple but reliable solution, using tried and trusted techniques and tools employed by numerous administrators over many years.
Any backups we take need to be stored on backup media. The most convenient solution is to have a spare Linux machine, with a number of hard drives, networked to our e-mail server, preferably located in another building. Storing backups off site is essential if we want to protect ourselves from a catastrophic event such as fire.
If a remote server is not available, an alternative could be some hot-swappable external hard drives connected to the server, or even, in a pinch, a DVD burner. Magnetic tape drives...