Erpnext uses the leave applicant's email address

Under Setup/Bulk mail I see that the system try to send email as applicant’s email. Is there any chance to stop the system using different email addresses? I have a default sending account atm which works perfectly.

I see that Error
(553, ‘5.7.1 Sender address rejected: not owned by auth user.’, u’’)

Prima Facie, it has nothing to do with ERPNext but your Mail Server.
It seems to be an issue with your Mail Server because, maybe you have setting enabled such as Strict User Auth.
Which prevents unauthorized users from using your mail server for unsolicited mailing.

Either you add the username / email address configured to send emails from ERPNext to your mail servers authorized users list or change it in ERPNext which have such right to send out email.