Hi Joe,
Are there any details showing in your Delivery logs for the messages that are getting through? I tested this on 16.3.6775, and I'm not able to replicate. The message never reached my inbox, and the Delivery logs show:
[2018.07.26] 14:49:28 [85162] Delivery started for [user]@smartertools.com at 2:49:28 PM
[2018.07.26] 14:49:34 [85162] Delivery for [user]@smartertools.com to admin@[domain].com has completed (Deleted) Filter: Blocked Domains
[2018.07.26] 14:49:37 [85162] Delivery finished for [user]@smartertools.com at 2:49:37 PM [id:7285162]
Could you try an upgrade to the latest public release and see if this resolves the issue?
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com