I'd like to be able to grant permission to a particular Security Group to be able to create/alter the SMTP addresses for a specific mailbox. Happy for this to be done via GUI or via PowerShell. I can find nothing about this level of granularity through Googling.
From here: https://technet.microsoft.com/en-us/library/dd638132(v=exchg.150).aspx
To manage Email addresses, you need to be part of the Exchange roles below. -Organization Management -Recipient Management -UM Management
From the above link, each of these roles has a hyperlink for further information.
http://exchangeserverpro.com/manually-configuring-email-addresses-for-exchange-server-2013-recipients-using-powershell/ is a good article on how to modify email addresses. From the ExchangeServerPro article:
Adds an additional email address without removing the existing ones.
To check which Management Roles you have access to, use:
https://technet.microsoft.com/en-us/library/dd351024(v=exchg.150).aspx and https://technet.microsoft.com/en-us/library/dd335173(v=exchg.150).aspx might also be what you are looking for.
http://www.codetwo.com/admins-blog/how-to-manage-role-based-access-control-in-exchange-2013/ also has a comprehensive document on Role Based Access Control.
Hope this helps.
Thanks, Tim.