If you are using SendGrid with Mailster you can also automatically progress bounces. You only need

On the delivery tab, you can find two options for handling bounces

Select SendGrid and future bounced email addresses will automatically get marked as hard bounced.

If you select Mailster you have to contact SendGrid Support and request to enable “preserve headers” option for your account. This step is required otherwise SendGrid will overwrite the return-path of your emails with their own.

You cannot use the WEB API when you handle bounces within Mailster!