The hiring company might be having a favorite software that they do not want to shift from and probably this might be the reason for this email marketing interview question. The motive behind the question is mainly to check whether you are known to that software or any other such software. However, the question does not have a fixed answer and so, it is better to be real with the answer. You should name the ESP you have already worked with, and finally suggest some other names that you would like to work with and will learn throughout the journey. This will help you lay a strong approach and keep yourself on a safer side.