Hi,
There is no reason to show the user who submitted the form an error even if email is not configured correctly because the message is received and can be viewed under View Messages regardless of whether the email is configured correctly.
Spoofing the entered address as the from address would be a bad idea because that is what spammers do and it can make the email land in the spam box and tarnish the reputation of the smtp server that relays the message.
We do use the entered email address as the "Reply To" address, in some email clients this works fine and you can click reply and it will use the reply to address while some email clients don't do this so we include the email at the top of the body of the message so you can paste it into a reply if needed. The from address is the from address listed in Site Settings Email From address, and generally it should be the same as the email user configured in smtp settings or at least an email address using the same domain as the one in smtp settings.
Hope it helps,
Joe