Hi Enrico
Yes, it is a Danish postmark. It is a socalled "Duplex"-type, which is a combination of two rings - one with letters (left ring) and one with a number (right ring).
I can translate "S.J." in the left ring, which means the postmark is "Sjællandske Jernbane Postekspeditionsbureau" (postmark used for railroad transport from Copenhagen to Korsør)0
It is very difficult to determine the number in the right circle. If the number is not "181", the postmark is from 1870 or later - I dare not say.
The letter ran from Copenhagen via Dampfschiff to Lübeck, to its final destination.
Cheers,
Ole