Rejected relay attempt Error

Hello everyone, I just had a few bad days after migrating about 200 websites from a 5 years old server, to a brand new one that uses Centos 7.6. I don't know if this issue really can't be solved by a setting in WHM/EXIM but the guys in the datacenter couldn't provide a solution to my clients issue. I have many clients that use their EMAIL CLIENT with SMTP set on port 25 or 26, Connection security set to NONE, and Password transmited unsecurely. Like in the image bellow. Now, with the new server this setting is generating an error when the EMAIL CLIENT (Outlook, Thudnerbird, Etc..) sends an email to a different server, like yahoo .. or any other different server! But if the email is sent to any account on the same server, there is no issue, the email is sent right away trough port 25 or 26 with password sent insecurely. With insecure settings if I look in the log file of the server I can see this error message (the email client is set to make an SMTP autentification) 2019-07-15 14:17:39 SMTP connection identification H= A=94.....50 P=49741 M= U=lucisopa ID=1072 S=ov***a@s***l.ro B=recent_authed_mail_ips_domain 2019-07-15 14:17:39 H=([192.168.1.2]) [94.****.50]:49741 F= rejected RCPT : Rejected relay attempt: '94.52.20.50' From: 'ov***a@s***l.ro' To: 'o***a@yahoo.com'
And with the mail client set on port 26 or 587 with Connection security : STARTTLS, the same email, the same recipient, is sent just fine to yahoo inbox. 2019-07-15 14:29:42 1hmzAq-0000iD-95 H=([192.168.1.2]) [94.88888.50]:49883 Warning: "SpamAssassin as cpaneleximscanner detected OUTGOING smtp message as NOT spam (-1.0/60)" 2019-07-15 14:29:42 1hmzAq-0000iD-95 <= ov****pa@si**ul.ro H=([192.168.1.2]) [94.***.50]:49883 P=esmtpsa X=TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=no A=dovecot_plain:eek:***pa@s***ul.ro S=922 id=8e59c82d-6172-61de-5dd9-99d570a2100c@si***l.ro T="testing from s***l.ro" for o***a@yahoo.com 2019-07-15 14:29:42 1hmzAq-0000iD-95 Sender identification U=l****a D=s****l.ro S=o***pa@si***l.ro 2019-07-15 14:29:42 1hmzAq-0000iD-95 SMTP connection outbound 1563190182 1hmzAq-0000iD-95 s***ul.ro o***pa@yahoo.com 2019-07-15 14:29:47 1hmzAq-0000iD-95 => o***a@yahoo.com R=dkim_lookuphost T=dkim_remote_smtp H=mta5.am0.yahoodns.net [98.137.159.25] X=TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128 CV=yes C="250 ok dirdel" 2019-07-15 14:29:47 1hmzAq-0000iD-95 Completed
So my question again, there really is no setting I can make on this new server, to let my clients use the same software they are used to ? On port 25 and 26 using no connection security? PS. I haven't opened a new ticket to cPanel support yet. Thank you.