"Traditional methods [of ensuring e-mail recovery and continuity] carry heavy facility, hardware, software and personnel requirements, and the capital and operating expenditures can be significant," Forrester writes. "Cloud-based message continuity offerings are a compelling alternative because they offload cost and responsibility to a provider."

Subscription pricing models are based on the monthly cost for each mailbox (usually $5 to $10) and the cost of storage (usually $3.50 to $6 per gigabyte), according to Forrester.

In addition to security and resiliency, there are numerous factors that could affect your choice of vendor, the report says. For example, levels of automation for failover can vary from administrator-initiated processes to a completely automated failover. Failback capabilities are also important, because "once the incident requiring failover has passed, you need to bring back messages that were sent and received during the failure," Forrester says.

When evaluating services, note that "there are two distinct capabilities: continuity of service and data protection," Forrester says. "For continuity of service, the offering can range from a Webmail interface for sending and receiving mail during an outage to adding recent (e.g., last week or month) or all messages. For data protection, a cloud provider could either augment or replace multi-site on-premise backup infrastructure."