i found this error.
This is the Postfix program at host mail.iscaa.com.
I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to <postmaster>
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The Postfix program
<firstname.lastname@example.org>: host mxs.mail.ru[18.104.22.168] said: 503 valid RCPT command
must precede DATA (in reply to end of DATA command)
what does this error means?