When are custom thresholds worth it for Image sources resolve?
When are custom thresholds warranted?
Set `approvedAssetHosts` when you want images served only from places you control or have vetted. It is worth doing when brand assets have been scattered across ESP-hosted uploads, agency servers and personal cloud storage, because each of those is a future broken image with a different owner. It also has a security dimension: restricting hosts limits what a compromised or expired third-party domain can put inside your email.
When should you stay on Standard?
Stay with the empty list while you genuinely do not know where your assets live, because a host list assembled from guesswork will fail correct campaigns immediately. Keep it empty when auditing mail you did not build — an agency's proof, a partner's co-branded send — since their hosting choices are not yours to fail. If your only concern is broken and insecure sources, the default already covers it and adding a list buys nothing.
What do you trade away?
A configured host list means every legitimate new host is a failure until someone updates the list, which is friction you must be willing to own — and the temptation in a hurry is to add whatever appeared rather than to question it. Too permissive a list gives the appearance of a policy with none of its effect. Note also that adding a host list does not make the broken-source checks stricter; it adds a separate policy layer, and confusing the two leads people to think a passing host list means the images render.
Community Standard vs org Custom
The Standard behaviour is written to be useful for everyone with no setup, which is why host restriction is opt-in rather than default. An approved-host list is your organisation's asset-hosting policy and belongs with whoever owns brand assets and security review, not with an individual campaign. Review it when a platform is retired, because a list still naming a decommissioned CDN both fails good mail and hides where assets have actually gone.
Worked override examples
A brand consolidating on its own CDN sets `approvedAssetHosts` to that domain alone, so any campaign still pulling images from an old ESP upload or an agency server fails until the asset is moved. A company working with two agencies lists its CDN plus each agency's approved delivery domain, keeping the policy enforceable without blocking live work. A team auditing a partner's co-branded send leaves the list empty, since the useful findings there are broken and insecure sources rather than a hosting policy the partner never agreed to.
How does an agent call it with parameters?
{
"type": "email",
"validations": [
{
"id": "email-image-source-integrity",
"parameters": {
"approvedAssetHosts": []
}
}
]
}