Create a shared mailbox in Exchange

New-Mailbox -Shared -Name "Internal IT" -DisplayName "Internal IT" -Alias InternalIT

Or convert existing user mailbox to shared mailbox

Set-Mailbox internalit@example.com -Type shared

add-mailboxpermission -identity internalit@example.com -user EXAMPLEinternalit -accessrights FullAccess
add-adpermission -identity internalit@example.com -user EXAMPLEinternalit -extendedrights Send-as