Hi Andrew,
Many thanks for your continued support. The webmail application that I was using my development virtual machine was called RainLoop. It was a very nice app. However it applies a lowercase transformation on outgoing email addresses. I've lodged an issue with the project maintainers and they are working on a solution.
In the meantime I've switched to Roundcube, and I can confirm that everything is working as expected. The issue that I had was caused by the email address being transformed to lowercase.
I'll continue my investigations and hopefully have a patch soon for this MDL-50811 tracker item, which started me down this path.
With thanks.
-Corey