There are a number of options to email certificates. 
Click the information icon next to the certificate number on the Home screen. Scroll down to Email-options and click to open the section.

You can send the PDF to yourself, to an administrator (if you have one), or direct to your client. If you use this option, the email will be sent fromus and will look like the image below. 

If you prefer, you can choose to send the email from your own email program. Click the Send from My email button, this will launch your email program (such as Mail or Outlook). You can now add your own text to the body of the email, attach other files such as your invoice.