Since the latest upgrade to 4.3.6, my cart cannot send mail. I get bounced email notices when orders are received, and when updating order status and selecting to notify the customer, I get this error:
ErrorMessage could not be sent.
Mailer Error: Could not instantiate mail function.
Edit: I received an email notifying me of the rejected emails that said this:
A message that you sent was rejected by the local scanning code that checks incoming messages on this system. The following error was given:
This message was classified as SPAM and may not be delivered.