First: The plugin is great!
Second: The currently only thing I find less professional is the view online and redirect links, as they’re exposing the usage and location of WordPress (folders and files).
Besides the problem of exposing WordPress folders (sometimes named in one specific language) and files, it would also look nicer and more professional if the redirect and view online etc. links were somehow based on the defined newsletter page (in the respective language, if more languages are used).
Examples:
domain.xy/en/newsletter/view/idxyz
domain.xy/de/newsletter/redirect/linkidxyz
Hope you find my idea useful, and that this is somehow technically possibble!