In principle, you should be able to prove that the recipient gave permission to receive the newsletter.

Having said that, the way in which the permission was obtained is less significant (written, by telephone, by web contact form), as long as it can be proven if needs be.

A difference between private and business customers only exists in a few very specific cases described in §7 para. 3 no. 1 to no. 4 UWG (Unfair Competition Act – Germany only) (briefly put: „if similar goods or services are being offered”).

More detailed information can be found here (available only in German).

A good general overview can be found here (available only in German).