"Error 550 5.7.708 Access denied, traffic not accepted from IP" when sharing link from Sharepoint

Dear all, 

I've encountered an issue on SharePoint - when I'm trying to share a file link via SharePoint, the message gets undelivered and received a NDR as per title 

Generating server: HK0PR01MB2196.apcprd01.prod.exchangelabs.com

<recipient email>
Remote Server returned '550 5.7.708 Service unavailable. Access denied, traffic not accepted from this IP. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653 AS(8561) [HK0PR01MB2690.apcprd01.prod.exchangelabs.com]'

Original message headers:

Received: from HK0PR01MB2196.apcprd01.prod.exchangelabs.com
 ([fe80::a874:cd68:fb56:1f38]) by HK0PR01MB2196.apcprd01.prod.exchangelabs.com
 ([fe80::a874:cd68:fb56:1f38%4]) with mapi id 15.20.1250.032; Thu, 25 Oct 2018
 02:54:42 +0000
MIME-Version: 1.0
Content-Type: text/plain
Date: Thu, 25 Oct 2018 02:54:42 +0000
Message-ID:
	<HK0PR01MB2196BCD45227A9664F44738BEFF70@HK0PR01MB2196.apcprd01.prod.exchangelabs.com>
Subject: User shared "<document name>" with you.

I've tried sending to different email providers and domain, and from reading some past posts, it seemed like the issue is pertaining to Office 365 / Exchange rather than on the recipient end. 

Sending email via outlook works perfectly fine, but only undelivered when shared through SharePoint. 

Appreciate any assistance on this issue urgently. Thank you ! 

Regards

Kenny