Hi there! Welcome . Are you talking about using forms on an external server? Or email notifications? Maybe both?
I have always used webflow’s hosting so I haven’t had this issue personally. It is my understanding that forms don’t work on externally hosted sites. You need to you another option for forms in that case. I’ve used both typekit and sumo forms in the past.
Support for remote form processing was eliminated some time ago. Last year on May 25, 2018 to be exact. If you exported the site before that, then forms worked until July 31st, 2018.