Ahhh good ole Microsoft sometimes making the smartest people feel dumb and everything in between. I was sitting at my desk trying to figure out why I couldn't map to a share drive in 2016. Come to find out I forgot to give the security group permissions only Domain Admins had permissions. I was like SOB of course, luckily it was only a 10 - 15 minute gaff on my part.
Man I don't know how you do it, I hate managing Exchange, shied away all my career then got it
dumped on me here in a sink or swim capacity. Exchange crashed and everyone looked at me, I looked down the line and noticed nobody else started after me.
Are you using 2012 or have you migrated the 10 sites to 2016 with Windows 10?