In my own opinion, Its worth it. Your email is a constant reminder that they signed up for SFI. If they decided to open the e-mail and read, they may have a rethink and choose to try it out. We have the testimony of some people who became active after a long period of inactivity due to the regular e-mails that were sent to them.
Do not be discouraged, continue sending them e-mail. Its going to be worth it at the end of the day.
