Design issues with WP Signature

Dear friends,

Do you have any experience with the WP Signature Plugin by ApproveMe?
We encounter the issue that apparently the plugins generated email which uses your default design is been received with design failures like buttons in wrong colors or not readable text.
Do you have an idea how to solve it? Or in the worst case is there a possibility to only exclude this plugin from using your default email template?

Thank you very much for any ideas!

Jan