My comment below was in response to this FAQ question
https://support.zendesk.com/hc/en-us/articles/226467267?page=1#comment_360004003594
I understand that through the GUI in Zendesk that this light-agent can not reply to a ticket, but through regular Email the ticket should be updated as normal.
I even tried to add the Light Agent to the CC list. but this did not change the behaviour.
We have given our Sales team access to tickets as light agents but if they cannot reply to the emails it is useless to us.



Light agent's reply via email are designed to be converted to a private comment, as what Phoebe mentioned.