Transactional email, or newsletter?

During complaint procedures, the question arises again and again as to whether the mailing in question is considered a transactional email or perhaps a newsletter. Answering this question can have a particular impact on the legal requirements to be observed for sending. The strict legal requirements for email marketing1apply not only to classic advertising mails in which certain products are explicitly advertised, but also to the sending of newsletters. So, when is an email “just” a purely transactional email, and when is it already considered to be a newsletter?