Message logs are optimized to maximize log retention, in order to achieve the maximum retention email logs go through a number of stages - Full Logs with message archive, Full Logs, and then Metadata. 

0-7 Days

Full Logs with archive are retained for up to seven days depending on your plan. During this time the message can be retrieved from Quarantine, downloaded, or retried. 

7-14 Days

Attachments are stripped but the full message logs including all retry information, delivery, and message disposition are retained. This is 14 days. 

15-45

Message logs are stripped and message metadata is retained. Metadata includes From / To / Subject / IP / Result / Spam Score but does not include the full transactional message logs or the message attachments.