When customers receive an email with a suspected virus, they often ask “What do I do now?

This blog post helps answer that question and guides you through our recommended process. This is intended for customers using Office 365 or Exchange Online Protection with on-premises Exchange servers.

First, it is important to understand the difference between an infected and uninfected email. Any email that has an attachment containing a script or malicious executable is considered ‘a virus’ for our purposes. This does not include subscription-based messages with links to malicious sites. Those messages would be considered spam and not viruses, and a different approach is used for spam messages (see this and this).

To deal with an email virus, here are some quick actions that you can perform:

1. Start at the filtering layer. We recommend using EOP, as it is the default option for Office 365 users. However, if you are using a third-party filtering mechanism, you will need to contact your vendor to investigate further. If the email virus has gone through Exchange Online Protection (EOP), or if header information is missing, then proceed to the next step.

2. Create a Transport Rule to block the message. Note that Office 365 Small Business and Small Business Premium customers will not have this feature access as mentioned here. See here also for information about transport and inbox rules limits. If you are an EOP customer, you can perform these steps as mentioned in Microsoft Knowledge Base article 2959596:

  • Navigate to Mail Flow in the Exchange Admin Center.
  • Click + to create a new rule.
  • Click More Options.

Under Apply this rule if, choose any attachment has executable content. At this point you can also choose an action under Do the following. We recommended choosing Block the message…

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Make sure no other Transport rules exist that would override this rule. See Manage Transport Rules for more information.

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3. Submit the email virus sample immediately to Microsoft Malware Protection Center (MMPC) for further analysis. In order to receive analysis updates, please sign into the MMPC Web site, or enter a valid email address. We recommend you to use your Microsoft account email address.

4. Once you have logged in, select O365 and Exchange Online Protection. Follow the instructions outlined on the MMPC to understand if you need to compress the email virus sample before uploading it to the site. Once you have completed the procedure, make note of the final MMPC ID that will be sent to you from the MMPC Submission Support Team.

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If you are dealing with an email virus that has a sender of administrator@domain.com or fax@domain.com with domain.com being same as your Office 365 domain, we also recommend blocking the sending server’s IP address and enabling the SPF hard fail in addition to the steps mentioned above. Also, see Best Practices for Configuring EOP.

If you continue receiving infected messages or attachments, then you should copy the message headers from the email virus, and contact Microsoft Customer Service and Support for further assistance. Be sure to have your MMPC ID handy, as well.

Irol Pinto
Technical Advisor, Microsoft Corporation