Absolit Consulting maintains a list of German email marketing agencies and their relationships to the most common email service providers (see also the ESP worldmap). Please find it in the two articles More Success with Specialized Email Agencies and Specialized Email Agencies Part 2. Both are in German.
The list would fit well into the agency-branch of the email marketing ecosystem. However, to make the visualization not more confusing than it already is, here’s a separate infographic, mapping the relationships between German email agencies and their supported ESPs:
Hints: Drag around a node horizontally while holding down the left mouse button to better see its connections. In addition, hovering over a link will display source and target nodes plus the link weight*. Alternatively, you can also download a static PDF for your records.
*The link sizes (=weights) are determined by the number of providers that the source agency supports. I’m assuming an agency is more important in the email space, if it offers help with many platforms. As a result, email service providers that have several links from bigger agencies may appear larger than those, which got even more links, but mostly from small specialized agencies.
When applying this weighting scheme, my employer optivo is most important in terms of agency relations. Overall, it has the 2nd most links. While @optivo ranks #1 among ESPs, the 1st place on the German email agency side goes to @erabbit, followed by @netnomics #infographic.