Microsoft released a patch for this issue, that was later incorporated into the cumulative updates. For further information see https://support.microsoft.com/en-us/help/2938053/third-party-transport-agents-cannot-be-loaded-correctly-in-exchange-server-2013
GFI MailEssentials fully supports Microsoft Exchange 2013 SP1. However, the latest cumulative update is required to avoid issues with the installation of Exchange agents.
For more information see https://technet.microsoft.com/en-us/library/jj983803(v=exchg.150).aspx