Lync 2013 and Exchange disabled accounts

Hi,

I'v found some Lync & Exchange disabled accounts is Users OU, what do they do? Is it safe to delete them or move to another OU?

 CN=Exchange Online-ApplicationAccount,CN=Users
 CN=SystemMailbox{bb558c35-97f1-4cb9-8ff7-d53741dc928c},CN=Users
 CN=Migration.8f3e7716-2011-43e4-96b1-aba62d229136,CN=Users
 CN=LyncEnterprise-ApplicationAccount,CN=Users

And this accounts is disabled, moved to another OU but still have mailbox:

DiscoverySearchMailbox {D919BA05-46A6-415f-80AD-7E09334BB852}

Thanks.