When you send mail using theMail API,there are some restrictions on the headers that you can use and the types offiles you can attach.

Optional headers

An outgoing email has headers that provide details about the message, such asthe sender, recipient, date, and subject. Each header has a name and a value.You can also include the following headers in an outgoing email; for securitypurposes, other headers are not allowed:

• In-Reply-To
• List-Id
• List-Unsubscribe
• On-Behalf-Of
• References
• Resent-Date
• Resent-From
• Resent-To

Mail with attachments

An outgoing email message can have file attachments. Each attachment has afilename and file data. The file data can come from any source, such as anapplication data file or the datastore.

Filename extensions that are prohibited

As a security measure to protect against viruses, email attachments cannot usethe following filename extensions, and ZIP files cannot contain files with thefollowing filename extensions:

• ade
• adp
• bat
• chm
• cmd
• com
• cpl
• exe
• hta
• ins
• isp
• jse
• lib
• mde
• msc
• msp
• mst
• pif
• scr
• sct
• shb
• sys
• vb
• vbe
• vbs
• vxd
• wsc
• wsf
• wsh

Attachments and Multipurpose Internet Mail Extensions (MIME) types

The MIME type of an attachment is determined from the file's extension. The MIMEtype identifies the type of content that is being sent, and can determine howthat data is transferred.

The following is a list of MIME types and their corresponding filenameextensions. You are not limited to these extensions, but if you use an unknownextension, App Engine will assign it the MIME typeapplication/octet-stream.