If you’re seeing the following message in Outlook. “The server responded 451 Greylisted…” Below is how you can fix the error.
Step 1: Go to your Account Settings
Step 2: Click More Settings > Outgoing Server > Check My Outgoing Server (SMTP) requires authentication > Use same settings as my incoming mail server
Step 6: Click OK > Next > Finish
At this point you try sending out a message. You should no longer receive the following message “The server responded 451 Greylisted…”.