Ensure you provide a valid email address as a return address in the From field of the email form.

Otherwise it may not be possible to send the email, as many email systems are very sensitive to emails without senders.

And, of course, it will be difficult for the recipient to reply to you. 

In some scenarios it may be appropriate for this to be a Form Field, however to ensure delivery the From address should be a fully qualified email address from a domain with a Reverse DNS entry ^(?).