http://searchexchange.techtarget.com/answer/Why-keep-that-server-after-an-Exchange-Online-migration?track=NL-1810&ad=919031&src=919031&asrc=EM_NLN_88695568&utm_medium=EM&utm_source=NLN&utm_campaign=20180131_Labels%20help%20admins%20with%20compliance%20in%20Office%20365