Random -,,Remote(SocketError) for App Relay

Hi team,

On my exchange 2013 server there is a receive connector configured for app relay server (out of many). This is able to connect and send mails but however, out of no where, the app server is unable to send mails at randomly. On the exchange SMTPReceive logs, I can see the below;

<EXCH>-MAIL1\SMTP Relay,08D6A23123A5E266,0,<ExchangeIP>:25,<SenderIP>:3915,+,,
<EXCH>-MAIL1\SMTP Relay,08D6A23123A5E266,1,<ExchangeIP>:25,<SenderIP>:3915,*,SMTPSubmit SMTPAcceptAnyRecipient SMTPAcceptAnySender SMTPAcceptAuthoritativeDomainSender AcceptRoutingHeaders,Set Session Permissions
<EXCH>-MAIL1\SMTP Relay,08D6A23123A5E266,2,<ExchangeIP>:25,<SenderIP>:3915,>,"220 <EXCH>-mail1.domain.com Microsoft ESMTP MAIL Service ready at Wed, 20 Mar 2019 23:04:57 +0830",
<EXCH>-MAIL1\SMTP Relay,08D6A23123A5E266,3,<ExchangeIP>:25,<SenderIP>:3915,-,,Remote(SocketError)

However, other app servers connections are successful at this time. 

1. I want to know why is this occurring?
2. Which end (app server or exchange) is causing the Socket error

Any clues would be great.

Thank you.

Jude.