Have been using the com.cividesk.email.sparkpost extension (v1.1) successfully on Civi 4.6.30 . Recently had an issue due to installing PHP mcrypt on the server, which caused the extension to be unable to retrieve the SparkPost API key. [More accurately: the call to
CRM_Sparkpost::getSetting('sparkpost_apiKey') was returning a scrambled value.] This was resolved, once the problem was diagnosed, by re-entering the API key. (I've written a separate Q&A about that issue, as it's not SparkPost specific.)
During the time when the API key couldn't be accessed, numerous transactional emails and CiviMails should have gone out. The CiviMails can be re-sent easily enough but what's the way to deal with the transactional emails? There's no record in the extension's log file of mails that attempted to send during this period. Is there any queueing system for failed messages? Because the API key couldn't be correctly retrieved, presumably the messages won't have been linked to the user's SparkPost account. The SparkPost stats show no messages over that period.