Attachments Not Getting Through
This article describes why some attachments don't get to the recipient and what you can do about it.
This article applies to: Outlook for Mac, Outlook for Windows, Outlook on the Web
The two most likely explanations are:
- The message was too large.
- The attachments were blocked by Office 365.
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.
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 Cmail accounts.
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.