Error on subscription / unsubscribe to category

I am experiencing an issue on michigandsa.org where users can subscribe to category, but no email is sent to the user or admin.


The subscription function seems to work OK, meaning the user is added to the subscription list for that category, and the appropriate dialogues appear.
On page reload, the error message "Notice Could not instantiate mail function." appears.
On clicking the "Unsubscribe to this category" link, the error message "Sorry, but the system is unable to load the subscription." appears.


Any help you provide would be greatly appreciated.
I am happy to provide site credentials, but I am unsure if this support request appears on the forums, and don't want to accidentally compromise the site.

Thank you!