After a bit more digging it appears that clamd is not running and throwing errors:
[root@mx ~]# clamd
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 497 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 512 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 528 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 544 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 557 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 603 undefined identifier "pe"
LibClamAV Error: yyerror(): /var/lib/clamav/antidebug_antivm.yar line 614 undefined identifier "pe"
LibClamAV Error: cli_loadyara: failed to parse rules file /var/lib/clamav/antidebug_antivm.yar, error count 7
LibClamAV Error: yyerror(): /var/lib/clamav/winnow_malware.yara line 65 duplicate identifier "CryptoWall_Resume_phish"
LibClamAV Error: yyerror(): /var/lib/clamav/winnow_malware.yara line 83 duplicate identifier "docx_macro"
LibClamAV Error: cli_loadyara: failed to parse rules file /var/lib/clamav/winnow_malware.yara, error count 2
[root@mx ~]# ps -A | grep clam
[root@mx ~]#
I have removed all of the databases from /var/lib/clamav then run freshclam to update - this downloaded 3 database files.
Overnight however the unofficial signatures were downloaded and the problem returned.
As a work around I have commented out the files listed above in: /etc/clamav-unofficial-sigs/master.conf
MailScanner should stop processing messages when clamd is not running as looking at the logs MailScanner seems to class all emails as viruses and deletes them as per the screenshot above - claiming "Other infection Y and MailScanner: Message attempted to kill MailScanner"