You can never be absolutely certain that the message will reach the recipient but there are a couple of ways to find out if it has. Before sending your message you can set it to ask for an automatic read and/or delivery receipt by clicking "Options" and selecting "Request a delivery receipt" and/or "Request a read receipt". Many email systems will honour this request and return an email to you to tell you that your email has been delivered to and/or read by the recipient but some do not and some people may not let their email system return a read receipt when they open your email. If it is important then it may be better to ask the recipient to acknowledge the email themselves. It's all a matter of judgement.