The solution will depend on if you want to keep the formatting or remove the formatting.
If you’re looking to remove all the formatting, you can use a find and replace statement - with Regex - on the text you want to use in the email body. I have used the following Regex and works in my instances \[[^]*?\]. Leave the ‘Replace by’ text empty to remove all the BB Code tags:
If you’re looking to keep all the formatting, you could look to do a find and replace for all the different BB Code tags and replace them with the relevant HTML tags. But this will also depend on how you’re parsing the text to Postmark. This solution will take some trial and error.