postmaster

It's about a picture of a knight in the Netherlands (XVII-th century).

The sitters can be identified by the coat of arms on the huge curtain: Jan-Baptista Anthoine, knight and postmaster of Antwerp, is shown with his wife Susanna de Lannoy,
two sons, Louis and Jan-Baptista, and two daughters, Maria-Alexandrina (1659–1723) and Barbara Catharina.

I wonder what is a postmaster here.

I can't see him as Webster's :
1 : one who has charge of a post office
2 : one who has charge of a station for the accommodation of travelers or who supplies post-horses

In this case, the postmaster would be the person in charge of overseeing all the postal mechanisms/institutions/offices/etc. for all of Antwerp.
In the USA, we call the highest postal official in the country the "postmaster general."

Still, for the 17-th century, it seems strange to me that the postmaster should be a knight.👁 Wink ;)

Dictionary.com includes the following definition:

postmaster 2. (formerly) the master of a station that furnished post horses to travelers.

Makes slightly more sense but we still end up with a knight doing a job!

Here :
« 13e congrès de l’U.P.U. », 14 mai 1952 : congrès de l’Union postale universelle, tenu à Bruxelles. Les 12 timbres représentent les grands maîtres des postes de la famille de Tour et Tassis et le château de Beaulieu ; d'après des dessins de William Goffin.

Turm & Taxis was (is) one of the great german families in Belgium.
So it seems right, finally. Thanks