App is sending all incoming SMS to email. Email sent through SMTP directly throught one of the mail servers - gmail.com, yahoo.com, hotmail.com, mail.ru, yandex.ru. So for app to work you need to have email account on one of these servers.
If internet is unavailable when SMS arrived, all such SMS will be kept by app and sent after internet connection is available.
(!!!) For some devices you need to manually set permission for app to read SMS. It is done by changing your device's OS settings.