In 2015 there was a issue raised in the Shiny Google Group about sendmailR not working on shinyappsio: https://groups.google.com/forum/#!topic/shinyapps-users/3PTjZ0dvehg
It still doesn't work. The on-screen message is "ERROR: An error has occurred. Check your logs or contact the app author for clarification."
The logs stop without an error message.
I have created a multi-page, multi-user skeleton for Shiny apps with user login. I need email to verify user email addresses.
Are there any workable options here, like a paid shinyappsio account or a different sendmail package?