The chosen channel is not valid for the following recipients

Hi,

I am trying to create a subscription for a document.

The problem is that when I select "Channel : E-mail Address" and click on "Save Subscription"

the error message occurs: The chosen channel is not valid for the following recipients.

When I select "Channel: Inbox", the subscription gets saved.

So the problem is with the Email Channel.

It appears that the email channel is configured (Channel, Channel originators, Subscription service), but why does this error message happens?

Regards,

Ladislav