Hi Will,

There has been a change in 18.1 regarding the sendMessage API at the end of the Release Notes for 18.1

Changes to Messages API - A new site-scoped role, Use SendMessage API, controls access to the LABKEY.Message API. Only users with this role can send messages via the API. Previously, read permission to a container automatically granted this level of access, so Guests or other groups might need to be added to the new role. Also, in a related change, the server no longer sends emails to users who have never logged into the server, except for the initial invitation email to set up their user account. (docs)

Regards,

Jon