rss reader with showing mails or newsletters as feed?

i heard there is at least one rss reader out there that can show mails or newsletters.
not sure how it works but it is possible to omit newsletters by mail, you can use theme directly in the feed reader.
hope for tips and maybe one of ours here can it and you use it.
give tips
thx
ps
i couldnt decide if general or apps section is better, done stone me