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Attachments Not Getting Through

This article describes why some attachments don't get to the recipient and what you can do about it.

The two most likely explanations are:

  • The message was too large.
  • The attachments were blocked by Office 365.

Message Size

The maximum size of a message, including all attachments, that you can send is 25 megabytes. Encoding the attachment (making it part of the message) adds to its size, so if a file is almost 25 MB, it may be too large when encoded. If you need to share files larger than 25 MB, you can use Cornell Secure File Transfer. This is a university-approved, secure way to transfer confidential or sensitive data.

Blocked Files

Outlook allows you to send any kind of attachment (up to the 25 MB per message limit), but blocks some incoming attachment types, specifically those types that could carry a virus or other malware.

Following widespread (reported globally but not yet at Cornell) "ransomware" attacks, we have begun blocking zipped files that include executable content (typically .exe or .com files) on both Office 365 and Google Workspace accounts.

What is ransomware?

May 2017 Ransomware Attack Wikipedia Article

The sender can get the file(s) to you in other ways:

  • Sender upload the file(s) to Cornell Secure File Transfer, recipient downloads them from there. (Members of the Cornell community only. Suitable for confidential and sensitive data.)
  • Sender uploads the file(s) to the Cornell Box cloud storage service. This should not be used for confidential or sensitive data.
  • Sender uploads the file(s) to any commercially-available cloud storage service. These should not be used for confidential or sensitive data.

List of File Types Not Permitted by Office 365

Some of the most commonly used file types are:

  • .exe and .com (executable files)
  • .bat (batch files)
  • .msi (installers)
  • .vbs (visual basic scripts)

Complete Listing of Blocked File Types
























































































































Don't recognize some of these file types? Neither do we! But you can Google them if you're interested.


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