Exchange 2013 - few mails are not reaching to users, they get lost.

Hi all,

I have exchange 2013 - 2 MB, 2 CAS server published through TMG 2010. internal as welll as external mails are very slow. few mails are getting lost also.we recently migrated from exchagne 2007 to 2013. Exchagne 2007 servers are shutdoen but not uet demoted. any suggestions would be highly appreciated on how to troubleshoot and which areas to check first.