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Announcement: changes to the way we handle headers when sending with SMTP

We wanted to let you know that on Monday, May 20th we’ll be making a change to the way we process SMTP headers for emails that are sent with SMTP. Please note that this change does not affect sending via the API.

Historically, when Postmark accepted messages over SMTP, we have constructed the To, Cc, and Bcc headers for messages sent to show only the subset of recipients you specify during sending. In some uncommon cases, this behavior meant that not all recipients would be visible in the message that is delivered to your recipients.

With this update, we will ensure that the recipients that are visible in your messages’ To and Cc headers are the same as what will be shown to the final recipients of your messages. We will no longer reconstruct them based on our older behavior, which, in some uncommon cases, could have prevented some recipients from being displayed in the final email that we deliver.

What you need to know:

  • This change only changes behavior for SMTP sending; API sending is not affected in any way.
  • We will never include the Bcc header in messages we deliver to your recipients. Your recipients will never see a Bcc recipient list in their email client.
  • For most senders, this change will not impact you. If you do not explicitly set the To, Cc, or Bcc headers during sending, then your SMTP client is building these headers based on the recipients for your email.
  • If you send messages to single recipients, this change will not affect you.
  • If you use Postmark to relay mail out of an internal mail server (such as Exchange or Postfix), the To, and Cc headers will now be displayed with all of the original recipients of that message, even if you are only relaying the message to a subset of the recipients via Postmark. Again, these headers will have been set intentionally by your mail client if you have included multiple recipients in a single message.

For the vast majority of our customers, we do not expect this change to affect what is displayed or sent from Postmark on your behalf.

To be clear, this is how recipient headers should work — we shouldn’t be altering these headers from your emails, so we’re correcting that. However, given the sensitivity of displaying recipients in email, we wanted to provide as much context as possible so that you can review your sending integration ahead of our release of this change.

Please let us know if you have any questions.

Rian van der Merwe

Rian van der Merwe

Product person. Lover of family, coffee, and vinyl records.