AWS Messaging & Targeting Blog

New Flexibility with Email Headers and Attachment Types

Amazon SES Senders: we’ve pushed important changes to improve your sending experience, thanks to your valuable feedback. Based on this feedback we know that one constraint that often created friction was the way SES approached validation on email headers and attachment types (file extensions).

Why do I describe the SES constraints on email headers and content types in the past tense? Because many of those constraints no longer exist!

Let’s take a look at the improvements that we’ve released:

  • Free form email headers – SES now allows you to use any header field name and value. The only constraints are your imagination and the header format specified by the RFC 822 standard. One thing to be aware of, though, is that some header fields do not make sense if they appear more than once in an email. The full list of headers that cannot appear more than once can be found here.
  • Opened up attachment types – we have compiled a list of files that we have chosen to disallow due to the frequency of use as an attack vector. This means that SES now allows you to send any file type that is not listed here, so you can send many more file types than you could before. In the end we want to be flexible, allow our senders and recipients to exchange great content types, while still preserving the recipients’ protection from common attacks. Please check back from time to time so you are aware of updates to the restricted list.
  • There are no longer constraints on Content-Type header values – we’ve eliminated any constraints related to type/subtype values. Just to remind you, type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Why is the removal of the Content-Type constraint so important? Because technically it complements the attachment types improvement by giving more freedom to specify the format of the attachment file. (We still encourage you to use standard rather than non-standard values.)

The changes described above are backward compatible and will not have any impact on your existing sending.

Before you take advantage of this new flexibility, make sure that the ISPs you plan to send to treat your headers as expected. This is important because each ISP has its own logic and constraints imposed on email types they accept.

We know that connecting with your customers is important to you, and these changes make it easier than ever for you to get the content you want to your recipients. For any suggestions or questions, go ahead and contact us! We invite you to post on the Amazon SES Forum. Thanks for using Amazon SES!