Postmark now have separated streams for broadcasts like newsletters or marketing emails, and transactional email. This is to optimise deliverability.
In order to use their services, they now require a header along with the API key, but AcyMailing only has the ability to add the API key in the configuration.
We now need another input, to add the Header: X-PM-Message-Stream.
If we don't have this header, Postmark will default to whatever stream is default (typically a transactional stream), meaning we can't use the Postmark integration until this is updated.
Are you able to add this, so we can use Postmark properly? See https://postmarkapp.com/support/article/1207-how-to-create-and-send-through-message-streams