How to Fix Outlook Attachments Blocked for Security Reasons in Microsoft 365

Summary

This guide helps you troubleshoot and resolve: How to Fix Outlook Attachments Blocked for Security Reasons in Microsoft 365. Follow the steps below to fix the issue.

Common Causes

Microsoft 365 includes built-in security features that block certain email attachments to protect your organisation from malware, phishing, and other threats. While these protections are essential, they can sometimes prevent legitimate files from being opened or downloaded in Outlook. This guide explains how to identify blocked attachments and the steps you can take to resolve the issue.

Quick Fix Steps

1. Check if the file extension is on Microsoft's blocked list (for example, .exe, .bat, .js, or .vbs).
2. Ask the sender to share the file via OneDrive or SharePoint instead of attaching it directly.
3. If you are using Outlook on the web, try opening the message in the Outlook desktop app or vice versa.
4. If you manage Microsoft 365 admin settings, check your organisation's Safe Attachments policy.

Detailed Instructions

Identify the Blocked File Type

Microsoft 365 automatically blocks attachments that are commonly used to spread malware. Common blocked extensions include:

• .exe, .dll, .com
• .bat, .cmd, .ps1
• .js, .vbs, .wsf
• .scr, .hta, .cpl

If the attachment has one of these extensions, Outlook will not allow you to open or save it directly. This is expected behaviour and cannot be overridden from the client side. Note that ZIP files containing these extensions are also blocked.

Request a Safer Delivery Method

1. Reply to the sender and ask them to upload the file to OneDrive or SharePoint.
2. Ask the sender to share a link to the file with you via email or Microsoft Teams.
3. Open the shared link and download the file from the cloud storage location.

Note: Sharing files via OneDrive or SharePoint is more secure than email attachments and avoids many attachment blocking issues entirely.

Try a Different Outlook Client

Some attachment restrictions apply only to specific versions of Outlook. If you are using Outlook on the web, sign in to Outlook on the web and check if the attachment is available there. Alternatively, open the message in the Outlook desktop app for Windows or macOS.

Review Safe Attachments Policies (Administrators)

If you are a Microsoft 365 administrator for your business, attachment blocking may be controlled by a Safe Attachments policy in Microsoft Defender for Office 365.

1. Sign in to the Microsoft 365 Defender portal.
2. Navigate to Email & collaboration > Policies & rules > Threat policies.
3. Select Safe Attachments and review the policy applied to your users.
4. Adjust the policy if a legitimate file is being blocked, then allow time for the change to replicate.

Warning: Do not disable Safe Attachments entirely. Disabling this protection exposes your organisation to malware and ransomware threats.

Troubleshooting

Attachment Appears but Cannot Be Opened

If the attachment is visible but you receive an error when opening it, the file may have been scanned and quarantined by Microsoft Defender. Check your Junk Email folder or ask your IT administrator to review the quarantine in the Microsoft 365 Defender portal.

Blocked Attachment in a Trusted Email

Even emails from known contacts can contain blocked attachments if the sender's account has been compromised. Never ask a sender to bypass security by renaming a blocked file (for example, changing .exe to .pdf). Instead, use a secure file-sharing method.

Outlook Desktop App Specific Issues

If attachments are blocked only in the Outlook desktop app, ensure your installation is up to date:

1. Open Outlook and select File > Office Account.
2. Choose Update Options > Update Now.
3. Restart Outlook and test the attachment again.

If you continue to experience issues with blocked attachments, you can Open a Ticket with 220 Internet Services for further assistance.

You may also find these related articles helpful:

• Setting Up an Email Account in Outlook (POP/IMAP/Exchange)
• Managing Email Signatures in Outlook
• Troubleshooting Outlook Send/Receive Errors
• Backing Up and Restoring Outlook Data Files (.pst/.ost)
• Organizing Emails with Rules and Alerts in Outlook
• Using Outlook Calendar and Scheduling Meetings

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