We discussed this with ESET Support, and their recommendation is to avoid using tag messages on all scanned emails. These tags can create a false sense of security. For example, malware could mimic such a tag on a malicious email to trick users into opening harmful attachments.

Additionally, a tag only reflects the email's status at the time of scanning. It’s possible that shortly after, ESET’s detection engine updates and identifies the email as malicious — but by then, the recipient may have already interacted with it under the assumption it was safe.

If, despite this advice, you still wish to add a disclaimer, the following outlines how it can be done. Please note that we do not recommend using the example wording below, as it may reinforce a misleading sense of safety.

"In ECOS if you go into Policies and either create a new policy or edit existing, then in the Email rules tab, you can create a new rule with the condition "Source" - "Scan Result", "Option" - "Clean" and then when you hit next and go into Actions, you can now (as of this week) select action "Add body banner". 

Now you can select if you want it to be before the text of the email or after, what color it should be and what it should say. You can even add characters such as ✓ into the banner, see image below.

As for deliverability against spam filters, I cant speak to that as it is a fairly new feature and we don't have the means to test it with every possible system out there, so I would advise you to maybe test it out for a short period of time on one account before a full roll-out to all accounts.