iCloud Email bouncing back when sent from new Macbook

I just installed my time machine back-up into this new Macbook so my iCloud accounts are synched (iPhone & Macbook).

For some reason when I send a message to specific ppl using Mail on my macbook with an iCloud account, the message bounces back and I receive the following:

Action: failed

Status: 5.7.1 (SMTP transmission failure has occurred)

Remote-MTA: dns;mx0.lcn.com (TCP|17.164.199.64|34022|94.126.47.85|25)

(mailguard-scan7.mailguard.com ESMTP CanIt-Appliance)

Diagnostic-code: smtp;554 5.7.1 MailGuard: Message rejected because of

unacceptable content. For help, please quote incident ID 08VBk7uIn.

But when I resend the "same message" with my iPhone the message is received/sent.

Can anyone help me on this one?