Using sp_send_dbmail with SQL authentication to send attachments from UNC path
I managed to get this working eventually in my Dev environment.


Enable xp_cmdshell
Add proxy account DomainLogin
grant execute on xp_cmdshell to SQLLogin
Grant logon as a batch job on Server 
On server open Secpol. > Security Settings > Local Policies – User Rights Assignment node. Add DomainLogin
Create SQL Credential "CredForMail" using DomainLogin
Grant Credential to : ALTER LOGIN SQLLogin ADD CREDENTIAL CredForMail
ALTER DATABASE DatabaseName SET TRUSTWORTHY ON


The end result is that I have a non-sysadmin user that can send emails with attachments it has picked up from a share on a remote server.

I think the proxy account and Credential may be overlapping and both may not be necesary. I have tried so many things I am no-longer sure of the bare minimum required to make this work. (and that bothers me)

Because of the complexity to set this up, enabling features that could be seen as a security risk, I am going to back all of this out, and instead will separate out this application function and give it it's own domain account.