The first section in your email should be a quick summary to identify the issue or the request. This is the reason why you are sending the email in the first place. The intro should be concise and clear.
The intro is usually one or two sentences long, and after that, you will provide more information about the issue.
It is best practice to address one issue in an email unless the problems are closely related. For example, you will not discuss a project line change with an employee and discuss his leave request in the same email. It should be two different emails.
Thank you for joining the new product presentation this afternoon. It has been a great pleasure to explain to you how the new XXX software works.