PeerSpot users discussed what the most important features of a cloud email service were, and the consensus pointed to usability. This includes the service's reliability/availability, support, and migration/integration abilities as well as its compatibility with other applications. On top of the basic user friendliness of the cloud email, members want to see clean a web interface and solid security features in place, along with easy customization that contributes to a strong market perception...
The cost is fair, and you end up making enough in sales to pay for it.
First think about the value of your list. The current list of potential sales is huge alongside growing the list.