Undelivered Mail

I suspect that there are recipient's servers that blocks my emails. I receive the following email from mailer-daemon@mail.zoho.eu :

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error.

pantelis.sarantos@unipakhellas.gr, ERROR CODE :550 - #5.7.1 Your access to submit messages to this e-mail system has been rejected.

Reporting-MTA: dns; mx.zohomail.eu
Arrival-Date: Thu, 04 Jun 2026 16:18:36 +0300

Original-Recipient: rfc822;
Final-Recipient: rfc822; pantelis.sarantos@unipakhellas.gr
Status: 550
Action: failed
Last-Attempt-Date: 4 Jun 2026 13:18:48 GMT
Diagnostic-Code: #5.7.1 Your access to submit messages to this e-mail system has been rejected.
Remote-MTA: dns; mx1.hc829-89.eu.iphmx.com

Received:from mail.zoho.com by mx.zoho.eu
with SMTP id 1780579118352350.14717934634075; Thu, 4 Jun 2026 15:18:38 +0200 (CEST)
Message-ID:<19e92c92713.7d5f1baa630975.8565979623996808564@ergomechanica.gr>
Date:Thu, 04 Jun 2026 16:18:36 +0300
From:ERGO MECHANICA <contact@ergomechanica.gr>
To:"pantelissarantos" <pantelis.sarantos@unipakhellas.gr>
Subject:INTRODUCING ERGO MECHANICA
Content-Type:multipart/alternative;
boundary="----=_Part_1944724_1400668823.1780579116822"
User-Agent:Zoho Mail

Could you help me with this?