How Do You Fix Exchange Online Quarantine Issues for Sender Names with Special Characters?

Why Are Emails with Brackets and Ampersands Getting Quarantined in Exchange Online?

Understanding Exchange Online Quarantine Behavior with Special Characters

Exchange Online administrators should monitor email filtering configurations, particularly when sender display names contain special character combinations. Recent reports indicate that legitimate emails featuring square brackets combined with ampersands in sender display names—such as [Company & Partners]—experience automatic quarantine without recipient notification.

The Technical Issue

Exchange Online Protection (EOP) identifies these messages as potentially harmful and isolates them in quarantine for 15 to 30 days before automatic deletion. The affected emails bypass the recipient’s inbox entirely, with senders receiving no bounce-back notification or error message. This creates a communication gap where both parties remain unaware of delivery failure.​

The specific pattern triggers EOP’s filtering mechanism when sender display names contain:

• Square brackets [ ]
• Ampersand character (&) within those brackets
• Example format: [Business & Associates]

How Quarantine Functions in Exchange Online

When EOP flags a message, it moves the email to a centralized quarantine location accessible only through administrator intervention. Standard users cannot view these quarantined messages unless administrators configure quarantine policies to send notifications from [email protected].

Recommended Solutions for Administrators

Create Transport Rules: Establish mail flow rules that allow emails matching specific sender patterns to bypass quarantine filters. Configure conditions that recognize legitimate business names containing these character combinations.

Modify Anti-Spam Policies: Access the Exchange admin center to adjust filtering thresholds. Create exceptions for trusted sender addresses or domains that use standard business naming conventions with special characters.​

Implement Allow Lists: Add verified sender addresses to your organization’s safe sender list. This ensures future communications from these sources reach intended recipients without filtering interference.

Configure Quarantine Notifications: Enable quarantine notification policies through the Security portal (security.microsoft.com) so users receive alerts when messages require review.​

Character Limitation Context

Microsoft’s filtering system demonstrates known limitations with certain special characters. Similar issues affect apostrophes in email addresses, where the block list validator treats specific characters as invalid, preventing proper rule application. This suggests broader character handling constraints within Exchange Online Protection’s validation framework.

Testing and Verification

Administrators experiencing this issue should document specific sender formats triggering quarantine. Reproduce the behavior in controlled test environments before implementing policy changes. Access quarantined messages through the Security portal under Email & Collaboration > Review > Quarantine to verify patterns and release legitimate emails.​

Monitor your organization’s email flow for similar filtering anomalies, particularly with international character sets or industry-standard business naming conventions. Report persistent issues through official Microsoft support channels to contribute to platform improvements.