Thanks for the reply Roman.
I have checked this morning and the emails are still stuck in the queue (submitted last night).
As covered in the issues description above, I have checked the error log (no related issues) and also successfully sent an email using the same SMTP server, recipient and sender details using the System application (System > Email tab).
When I find time later today I was hoping to use dotPeek to debug the Kentico assemblies.
Does anyone have any other ideas?