Email Attachments

Customer.io doesn’t support attachments of any type in Campaigns or Broadcasts. But, there are other, better ways to send files to recipients!

Why you shouldn’t send attachments

Sending attachments can create problems for your deliverability. Here’s a non-exhaustive list of potential problems stemming from attachments:

  • Email attachments are often used to hide viruses and malware, so they are more likely to end up in spam folders or flagged as potentially malicious.
  • Some email servers have strict policies around message size limits and/or attachments which can cause messages to bounce.
  • Attachments impose a larger download for recipients, which can impact their data consumption and/or lead them to ignore your email.

All of these can damage your deliverability.

If you want to send a file to your recipients, the best approach is to send them a link to a landing page where they can download the file.

 Avoid sending a link to a file directly

You can send a link to the file itself, but be aware that some spam filters and algorithms have been known to follow links to evaluate them, and if the link leads to a file, it could be considered suspicious. Microsoft Office files are particularly risky.

While more convoluted than using a direct link, this approach helps ensure that your emails are not flagged as spam.

  1. You should host the file online—either on your own server or a cloud storage service like AWS S3, Google Drive, Dropbox, Box, etc.
  2. Create a page that links to the file. Your hosting service might already create one for you. Otherwise, you’ll need to add a page to your website.
  3. Copy that page’s URL and use it in your email.

File attachments with the transactional API

Our Transactional API does support attachments through an attachments parameter.

You can specify for this parameter a dictionary of attachments where the filename is the key and the value is the base64-encoded contents. The total size of all attachments must be less than 2 MB. Do note that some filetype extensions are restricted.

Copied to clipboard!