Users are reporting that Gmail won't sync mail from their Microsoft Exchange account. What could be the cause of this?

The most direct and common reason for a mail client like Gmail failing to sync with an Exchange server is an incorrect server address. If the server name, port, or security settings are misconfigured, the connection cannot be established. While an older device might not support the latest Exchange ActiveSync (EAS) 16 protocol, this is less likely to be the primary cause of a complete sync failure across multiple users, as EAS 16 is primarily for advanced features, and older versions would still allow basic sync. An expired enrollment token would prevent device enrollment, not affect mail sync on an already enrolled device. The EMM not supporting managed configurations is unlikely to be the sole cause for a basic mail sync failure, as many EMMs can push basic account settings even without full managed configuration support.