Mailer Error: Could Not Instantiate Mail Function

Store Builder Questions
1

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.
2

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:

We recommend to contact your server administrator regarding this.

As an alternative you can try using SMTP instead of PHP mail function