This has been fixed
Describes how MassTransit file purging works, and explains that MassTransit does not actually delete the files on the disk - those must be removed manually or via an automated script.
For information on how to delete files on MassTransit 7 Servers, please refer to the MassTransit 7 documentation.
With continued use of a MassTransit server files being transferred will accumulate in the Sent and Received folder within a user's mailbox folder.
Fixed in build 540