On Monday, May 20th we made 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
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’
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.
Read the blog post for full details.