There is no way to securely send an email as email is simply not a secure (or a guaranteed) service in any way at all and was never designed to be so it's inherently insecure.
You can encrypt the contents of the email so if it is intercepted it can't be read, but then you have to ensure that the recipient has a way to decrypt the message. if you google for encrypt email you'll find lots of information. this is typically easier to do using a proper mail client, such as outlook, rather than a web mail service.