I tried to add a "reply-to" header in mails form a "contact us" form.
Only "from" header is available, reply-to has the same value.

If "from" email set by user in contact form is unvalid, mail is not sent but user don't have any error message.
Set "reply-to" header with user mail and sender with EmailSender configuration value solve it.

I made some changes in kernel/content/collectinformation.php

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< $replyto = $tpl->variable( 'email_replyto' );
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< if ( !$mail->validate( $replyto ) )
<

{ < $replyto = $sender ; < }

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< $mail->setReplyTo( $replyto );
—
> $mail->setReplyTo( $sender );