Thanks for reminding me....I'll change that in the released version.
Due to the alterations I noticed that the TARGET_EMAIL is not in line m387 anymore but on line 428 (just for your documentation purposes)
The form itself still needs to specify the email address you want to send to.* The TARGET_EMAIL in formmail.php is a security check - a pattern that must be matched by the email address in the form so that spammers can't use your formmail.php as a spam gateway.
secondly I observed that that entered email - address (email) is not forwarded to the recipient, realname and mesg do so correctly but email of the sender is not processed correctly in the email message itself, maybe you can have a quick look to it
So, in your HTML form you need a line like this:
Note that it is "recipients" not "recipient" - you can have a comma separated list of them.
<input type="hidden" name="recipients" value="firstname.lastname@example.org">
Thanks again for using FormMail.