For more confirmation, you can send mails which request specific information from your affiliates. If you receive no responses at all, it is a confirmation that your mails are not reaching them. To ascertain that your mails are delivered, you may choose to use all sources including “live chat” online for confirmation. I must caution though that many affiliates as a matter of choice never want to be bothered with mails. Factor this fact in too when you are communicating with them.
SFI also provides “opt out” option for mails. It also has a tool to detect the number and rate of opt outs in your downline. You can use the tool to detect how many of your affiliates have opted out and the reason(s) why. That is one other sure way to confirm who is receiving your mails.