In the past couple of weeks, we've been having "Message Send Failed" error messages when trying to send New Order Receipt on some orders, but not all. A test order to one of our @kataaro.com email addresses did not result in this error message.

At first we thought it had to do with some spam filters the customers have setup. However, we now have a regular customer who cannot log in to their account and receive an error message when attempting to do so, which is making me second guess the issue.

When I went into their account, I clicked the Reset Password button, but received another Message Send Failed error message. We changed the email address on their account to addresses with different extensions (gmail, hotmail, etc) that belong to any of us, and the only one that worked for the password reset email was one of our emails with @kataaro.com.

What are some possibilities for this new issue? We have not yet updated to the latest patch.