There have been reports of large groups of email addresses returning as bounced or bad with the message, "Unable to resolve MX record for [domain.com]: servfail."
This message is typically an indication of a problem with a recipient's mailbox, but the reports note that a considerable portion of these email addresses were recently emailable. The problem does not appear to be limited to any particular ISP.
One reporter simultaneously saw an uptick of the error, "550 <[firstname.lastname@example.org]> recipient rejected. Recipient does not exist" for BigPond email addresses.
We are looking into this issue with our email provider. If you see any other additional examples, please email email@example.com.