I need to add a new customized mail action (to add some mail headers and other bits and pieces - see https://github.com/malariagen/alfresco-discussions/blob/master/discussions-email/src/main/java/net/malariagen/alfresco/action/CustomMailAction.java)
The problem I have is that if I add an action to the OutboundSMTP subsystem then it will break the default mail action.
The easy fix for this is to add the sourceBeanName to the mail definition in action-services-context.xml
It might be worth considering making the sourceBeanName mandatory since the proxy breaks if more than one class matches.
(I haven't raised an issue to improve the mail action even though my changes are generic)