The Postmark SMTP service may not be compatible with all SMTP clients. If the recipients are not submitted to Postmark in a single transaction, it can cause the recipients to be placed into separate messages. Specifically, this can occur when using a software client such as Gmail or Office 365 to generate the outbound emails.
We do not recommend using Postmark's SMTP service in these cases and instead use our API, if possible. It is on our roadmap to improve this behavior to accomodate more software clients in the future.