@nithinbabu The '' at the end of the notice is odd. How do you have the entry formatted, and are you adding it to the entry to Allowed Senders or Allowed Domains (anti-spam policy)? The only time I have received any sort of "Invalid address" errors is if I add a domain to the Allowed Senders list (needs to be email@example.com), or if I add a domain starting with @ (e.g., @contoso.com) to the Allowed Domains list (should just be "contoso.com"). Also, note that we recommend that you utilize other tools to allow senders into your organization, such as using the Tenant Allow Block List, spoof intelligence, or mail flow rules (which have better control over allowed senders), as the allowed domains list can be exploited. Here's additional background and recommendations: Create safe sender lists in EOP.