Storage
Business Standard: Includes a 50 GB mailbox
E3: Includes a 100 GB mailbox and unlimited storage with an in-place archive
Security
E3: Includes security features like message encryption, data loss prevention, and rights management
Basic: May not include security or compliance capabilities
Features
E3: Includes advanced data analytics, enhanced compliance tools, and unlimited archiving
Business Standard: Includes Exchange Online, but you can purchase an add-on for more storage
Basic: Includes core Microsoft productivity offerings, but may not include security or compliance capabilities
Other differences
E3: Includes Hosted Unified Messaging services, which provide call answering, a dial-in user interface, and automated attendant capabilities