Outlook Express is a freely available Mail User Agent (MUA) for Microsoft Windows Operating Systems. It is distributed and maintained by Microsoft.

It is possible to send attachments to Outlook Express users using non-standard attachment techniques. This can be accomplished by encapsulating the data in Carriage Return (<CR>) specifiers in the Subject line of an email. Upon receiving an email with a subject line containing carriage returns, Outlook Express will interpret the data section of the mail beginning in the subject line. This problem is compounded by the fact that mail filtering utilities do not search the subject line for this type of data, and can allow a malicious file to pass to an Outlook Express user.