Thanks for your reply Nikolay.
When I was using 11.5 I had a service running that by default used the sendmail binary to send me log notifications and it was working fine pre-upgrade.
The mail was being sent to an off-server email address without any problems. Immediately I upgraded I noticed the notifications stopped.
I have now set the configuration of the service to use SMTP and the notifications are flowing again.
I have to admit there are a few files not where I expect then to be - for example I can't find the mod_security configuration file - I had this installed pre-upgrade and uninstalled it before upgrade as per instructions - I felt managing mod_security via plesk would be easier but it seems that the Plesk version has it's config, log, and rules files in different places that I haven't been able to track down yet.