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...
Security, easy to use, support and customerization.
Security is the most important for me. Many cloud email providers do not use basic security encryption, so definitely go with one that is.
Ease of use, security, reliability, customization (APIs), and pricing models.
Usability, compatibility with other applications via plugins or API and it's market perception.
Reliability, web interface (GUI), security, availability, support, migration, integration, customization.
Friendliness to the end user.