When you send an email, there are tracking options to ascertain whether the recipient has successfully received the email, and seen the email, however the email recipient can opt out of providing you with this information or their email program may not support it.

A delivery receipt informs you that your e-mail message was delivered to the recipient’s mailbox, but not that the recipient has seen it or read it.
A read receipt informs you that your message has been opened.

You will receive this notification via an email, and if you see this icon on the outgoing message, you can open and CLICK the tracking button to see the status.
from Heather Smith ~ MYOB Certified Consultant