Customers have reported issues with Exchange Server 2013 Transport services not starting after the upgrade to Service Pack 1. You can see some of the reports in the comments of the Service Pack 1 release post on the Exchange team blog.
The problem is appearing on servers that have integrated 3rd party transport agents, for example antivirus software, disclaimer software, and fax software.
The problem lies with assembly redirection policies files being improperly formatted – they contain unsupported commenting style what makes them being unrecognized as valid XML and therefore ignored by Microsoft .NET. A solution is to remove invalid comments line. Two config files in your Global Assembly Cache folder contain unwanted comments.
The two files in question are:
Microsoft have published KB2938053 with a fix for this issue.
Because this problem impacts multiple vendors there may additional pre or post tasks required, such as removing the third party software first, or running additional steps after the server restart to repair the integration of the transport agent. Refer to your vendor support if necessary for further details.
Tony Redmond has published a more detailed version of the events surrounding this issue.