Microsoft Exchange Online

Exchange Online is the cloud-based email, calendar, and contacts service underpinning Outlook in Microsoft 365. It offers enterprise-grade reliability, massive mailbox storage, and advanced security features like anti-malware and anti-phishing protection. It's the default choice for businesses of all sizes using Microsoft 365, providing a professionally managed email infrastructure. It scores highly for its robustness, seamless integration with the Microsoft stack, and comprehensive compliance tools.

The main drawback is that its advanced administrative features and licensing tiers can be complex for small businesses to navigate.