Postfix cannot send any mail to internal domain
Solution 1:
Your recipient is rejecting the mail you sent because the sender email address cannot be verified as real.
Jul 26 22:47:03 mydomain postfix/smtp[15883]: 658DD400BC: to=<[email protected]>, relay=lx31.hoststar.hosting[168.119.43.209]:25, delay=0.51, delays=0.14/0.13/0.19/0.05, dsn=5.0.0, status=bounced (host lx31.blabla.hosting[168.119.43.209] said: 550-Verification failed for <[email protected]> 550-Unrouteable address 550 Sender verify failed (in reply to RCPT TO command))
To send email to this recipient, use a real email address as the sender.