Autoreponse goes to wrong Email address - php

Question

I have 3 mail ids say, user1#abc.com, user2#abc.com and user3#abc.com. I have setup PHPMailer with SMTP user as user1#abc.com. Now I set an auto response to user3#abc.com.
When I send a mail from user2#abc.com to user3#abc.com, autoresponse is going to user1#abc.com, the address i used to configure SMTP. How can I make autoresponse to send to FROM address?
$mail->isSMTP();
$mail->Host = 'hostname';
$mail->SMTPAuth = true;
$mail->Username = 'user1#abc.com';
$mail->Password = 'password';
$mail->SMTPSecure = '';
$mail->Port = 25;
$mail->Sender='user1#abc.com';
$mail->AddReplyTo('user2#abc.com', 'User2');
$mail->setFrom('user2#abc.com', 'User2', FALSE);
$mail->addAddress('user3#abc.com', 'User3');
$mail->isHTML(true);
$mail->Subject = $subject;
$mail->Body = $mailBody;
$mail->send();

Answer 1

You have not set a to address, so it's falling back to a default.
$mail->addAddress('user3#abc.com');
Many ISPs (for example gmail) will not allow you to set arbitrary from addresses, so if your from address is not working, check that. Also, be sure not to forge from addresses as it will cause delivery failures when you break SPF rules.
If you're writing an autoresponder, check that the inbound message you're responding to does not have a Precedence: bulk header set; autoresponses should not be sent to mailing lists (which should set that header) as it will often cause loops.