Get mailboxes with IMAP enabled:
Get-CASMailbox -resultsize unlimited | where {$_.IMAPEnabled} | select Name, SamAccountName
Create distribution list with no members. Emails sent to this DL will go into blackhole and won’t NDR.
Exchange silently deletes them.
Outlook 2010: Automatically Spell Check Emails Before Sendinghttp://www.addictivetips.com/windows-tips/outlook-2010-automatically-spell-check-emails-before-sending/