E-mail is definitely still a business critical tool, you just can't live without e-mail. However there are a lot of threats that make e-mail also a very vulnerable tool, you need to protect yourself against virusses and spam.
The best solution to protect your e-mail today is via the Ironport (in the meantime acquired by Cisco) mail firewall. This mail firewall acts as a mail relay with optional anti-virus and anti-spam protection. The unique SenderBase network is used to block spam mails as well as new virus outbreaks very accurately. Cisco is using a pro-active method in order to block spam by relying on the reputation of the IP address of the sender of the mail.