Strange Email Notification Issue on latest version of ABR11.5

Hi all,

*Note that some server names and email addresses have been changed to protect our infrastructure.*

We have the following running in our office:
on BACKUP = ABR Adv Workstation v11.5
on EMAIL = ABR Adv Exchange Server v11.5 and ABR Adv Server v11.5
on FILES = ABR Adv Server Linux v11.5

All are on the latest v11.5A_37975 version.

BACKUP has the following jobs (as a Management Server):
Backup files using Linux Agent on FILES to Centralized Vault c:\BACKUP\FILES-LINUX.
Backup files and Exchange mailboxes / stores on EMAIL to Centralized Vault C:\BACKUP\EMAIL-FILES and EMAIL-MAIL.
Backup C:\BACKUP to TAPE (LTO-5) via local Personal Vault on BACKUP

The issue we have is a strange one.

All the backups send email notification, with the exception of the Linux one. It fails with error the below logged on the local Linux Server Agent:
Message: Fatal error has occurred while reading from the socket.

and yet the email settings are IDENTICAL to all the other jobs, which work fine.

FULL LOGS ARE BELOW:
--------------------
Log Entry Details
--------------------
Type: Error
Date and time: 29/11/2013 12:53:24 AM
Backup plan: [None]
Task: [None]
Code: 20,250,685(0x135003D)
Module: 309
Owner: root
Message:
Command 'Sending e-mail' has failed.
Additional info:
--------------------
Error code: 61
Module: 309
LineInfo: 4a8728dc8a1c950f
Fields: $module : mms_lx_37975
Message: Command 'Sending e-mail' has failed.
--------------------
Error code: 32
Module: 335
LineInfo: da7e497cf4160e20
Fields: $module : email_commands_addon_lx_pic_37975
Message: Failed to send the message from 'usercompanydomain.ext.au' to usercompanydomain.ext.au' (SMTP 'mail.bigpond.com:25').
--------------------
Error code: 15
Module: 335
LineInfo: da7e497cf4160e0f
Fields: $module : email_commands_addon_lx_pic_37975
Message: Fatal error has occurred while reading from the socket.
--------------------
Acronis Knowledge Base: http://kb.acronis.com/errorcode/

Event code: 0x0135003D+0x014F0020+0x014F000F

--------------------

Appreciate any assistance to get this one resolved.

:)

NOTE: Test emails from FILES work without a problem. I've email notification settings up in the Default Backup Options for both the local machines/agents, as well as the management server.