"Couldn't block sender. We couldn't block this sender try again later."

Dear gatotten,

Thank you for reaching out to the Q&A Forum!

It sounds incredibly frustrating to suddenly lose the ability to block senders in Outlook, especially when you're diligently trying to manage your junk mail and hitting that "Couldn't block sender. We couldn't block this sender try again later." error. You're right to look for the root cause, and it's certainly a puzzle when something that used to work suddenly doesn't.

Let's break down what's likely happening, focusing on the most probable causes for this specific error, especially given your recent Windows 11 update and your method of trying to block by IP address.

The Most Likely Root Cause: Incorrect Format for Blocking

The primary suspect here is how Outlook's Blocked Senders list is designed to function.

• Its Purpose: This list is primarily built to recognize email addresses (like spammer@example.com) or full domains (like @example.com).
• The Problem with IPs: It's generally not designed to accept raw IP addresses. While some older Outlook versions might have silently accepted an IP address in that field (even if it wasn't truly effective for blocking), a recent update likely introduced stricter validation. This means Outlook now correctly identifies an IP as an invalid entry for that specific list and throws the "Couldn't block sender" error immediately.
• Why It Feels Sudden: This stricter validation, potentially pushed with your recent Windows 11 update or an Outlook update, could be the reason why a method that previously seemed to work (or at least was accepted) is now generating an error.

Other Possible Causes

 If the format isn't the sole culprit, here are other possibilities:

• Corrupted Junk Email Filter Settings or Outlook Files: A Windows or Outlook update can sometimes inadvertently corrupt critical files that store your Junk Email settings or even your main Outlook data file. This corruption could prevent any new entries from being added to your Blocked Senders list, regardless of format.
• Temporary Outlook Process Glitch: Sometimes, software just has a momentary hiccup. A small glitch in Outlook's internal processes for managing the Junk Email filter could be causing this specific failure. The "try again later" part of the message often hints at such a transient issue.
• Update Conflict (Less Common for this Specific Error): While your Windows 11 update is a strong contextual factor, the "couldn't block sender" message when trying an IP format is quite specific. However, it's always possible an update introduced a broader bug related to managing the Blocked Senders list.

What to Do Next: Pinpointing the Root Cause

Let's run through some targeted steps to confirm the exact problem and get you back to effectively managing your inbox.

1. Test the Core Blocking Functionality:
   • Find a spam email that has a regular email address (like ******@badmail.net) or a domain (like @badmail.net) as the sender, not an IP address.
   • Right-click on this email in your Junk folder (or inbox).
     • Select Junk > Block Sender.
     • What happens?
     • If it blocks successfully: This strongly confirms that Outlook's core junk mail functionality is working, and the issue is indeed specific to your attempt to block by IP address in that particular list. This means the format is the root cause.
     • If it also fails with the same "Couldn't block sender" error: Then we know the problem is broader than just the IP format. It points more towards corrupted settings or a general software glitch. Move on to step 2.
2. Address Corrupted Settings or Glitches (If Step 1 Failed):

• Restart Outlook (and your PC): A simple reboot can often clear up temporary software issues.
• Rebuild Outlook Search Index: This often helps resolve underlying issues with settings files. Close Outlook, then go to Windows Control Panel > Indexing Options > Advanced > Rebuild. Confirm, and this process may take some time. Restart Outlook afterward.
• Check for Outlook Updates: Even if Windows updated, there might be separate Outlook updates. In Outlook, go to File > Office Account > Update Options > Update Now.
  • Repair Outlook Data Files: Use the Inbox Repair Tool (scanpst.exe). Search for "scanpst.exe" on your computer to locate and run this utility, which is designed to diagnose and fix errors in your Outlook data files (.pst or .ost).

1. The Correct Way to Block by IP Address (If Needed):
   • If your core issue is that you need to block based on an IP address (which is found in the message header, not the sender's email address), the Blocked Senders list isn't the right tool. You'll need to create an Outlook Rule instead:
   • In Outlook, go to File > Manage Rules & Alerts.
   • Click New Rule.
   • Choose "Apply rule on messages I receive" > Next.
   • Under "Which condition(s) do you want to check?", scroll down and find and check "with specific words in the message header."
   • In the lower pane, click "specific words."
   • In the dialog box, paste the full IP address you want to block. Click Add > OK.
   • Click Next.
   • Select an action, such as "delete it" (or "move it to Junk Email folder" if you prefer). Click Next.
   • Add any exceptions if necessary > Next.
   • Give your rule a name, ensure "Turn on this rule" is checked > Finish.
   • This method directly targets the IP in the message header, which is how server-level IP blocking typically works.

I hope these steps provide some immediate relief and guidance. Please let me know what you find, and if the problem still persists then we can adjust our approach from there.

Warm regards, 

Kal Duong | Microsoft Q&A Support Specialist

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