The letters you receive can be answered if the sender in question has made this possible.
When you see your post, press 'Reply', which will be visible at the top of the page on the left.
Then you will start a new message where the letter will be attached.
If the 'Reply' button is not next to the document, it cannot be answered directly in e-Boks.
You will be able to select 'Write new post' or 'Forward' where you can select them as recipient.
If you are not allowed to choose them as a recipient, you must instead contact the sender in another way.
Does it cost anything?
Answering letters on e-Boks.dk does not costs anything, the same applies to using 'Write new mail'