Just a simple form:
- an input email
- a text area message
- a send button
When you click on the send button, it send the mail.
You have to use a PHP Backend.
- Use bootstrap OR Fundation OR Material design Light
- Check if the mail is valid (use REGEX)
- Limit the size of the message to 500 characters
- Allow emoji, bold text, images, youtube video
- Let user add several recipients
- etc..