Why do I see the message “The add-in SendGuard 365 has prevented the item from being sent” when sending an email?

This message typically appears when there is a temporary drop in internet connectivity while using the New Outlook experience. When Outlook detects a connection issue, it can automatically disable all add-ins, including SendGuard.

To fix this, close Outlook and restart it while connected to a stable internet connection. After restarting, the SendGuard add-in should load normally and you can resend your email.