Incompatibility with sparkpost module

As Mandrill shifts from free/cheap to expensive I went looking for an alternative. Enter the Sparkpost module.

But when I tried to use it with message_notify I began getting connection failures, apparently due to a newline character message_notify passes in the subject line. This is not a problem with Mandrill or with sending via mimemail or htmlmail.

Thanks to @zaporylie, there is now a patch at #2712857: Sanitize email subject → that sanitizes the message_notify header.