UD Exchange Online: Email Attachment is Missing or Unreadable


If someone sent you an email message with an attachment, but the attachment is missing when you receive the message, it may be because the type of attachment that was sent is not a permitted attachment type. You should verify that the attachment was a permitted file format. The following attachment types are blocked:

  • *.bat
  • *.com
  • *.ps1

In general, the following files types are also blocked:

  • Executable files (.exe, .dll, .ocs, .sys, .scr)
  • Self-extracting executable files
  • Java Archive format (JAR)
  • Microsoft Cabinet Archives
  • Java Bytecode format
  • Object Code format

Unreadable attachments

If the email you are sending includes an attachment envelope called winmail.dat, and this file cannot be ready by some recipients (usually non-Exchange users), there is a method for Outlook users to disable the proprietary winmail.dat attachment envelope, making their attachments readable by all recipients. Information on how to do so is available at:




Article ID: 489
Fri 7/26/19 2:52 PM
Tue 8/27/19 12:10 PM