Your recipients are getting a pishing warning because your emails are not passing SPF checks. That simply means that the from
domain you are using is not authorizing your server to send mail on its behalf.
Try using PHPmailer https://github.com/PHPMailer/PHPMailer
You can use it to set a separate from address as well as a specific reply to address
$mail->AddReplyTo('replyto@email.com', 'Reply to name');
$mail->SetFrom('mailbox@email.com', 'Mailbox name');
That being said. The SetFrom
address is an address that must pass SPF and preferably DKIM checks to be NOT marked as SPAM or PISHED. This address is recognized as the BOUNCE address where all the bounced emails will be returned to. Not having a valid address may possibly disrupt future deliveries.
The AddReplyTo
address will be loaded when the reply button is clicked by the recipient. Keep in mind that even though the message may pass SPF there are many SPAM filters that will potentially mark the message as spam.