Most organisations treat email security as something they complete internally.
They secure their domains, implement DMARC, clean up SPF records, and tighten authentication controls. Once this work is done, it is easy to assume that the most serious risks are under control.
What is often overlooked is how much business email activity sits outside the organisation while still carrying its authority.
Every day, employees receive emails from law firms, banks, auditors, payroll providers, and SaaS platforms embedded into core workflows. These messages are trusted by default. People act on them quickly because the sender is familiar and the relationship already exists.
That trust keeps the business moving, but it also creates exposure that is rarely managed with the same discipline as internal systems.
How trusted partners become the weak link
Breaking into a well-protected organisation directly takes effort. Targeting one of its trusted partners usually takes far less.
When finance receives an email from a known law firm or bank, the assumption is that the message is legitimate. The recipient responds to context and urgency, not to authentication details they cannot see.
If that partner has not enforced DMARC, spoofing their domain is straightforward. The email passes security checks because the domain is recognised and permitted. It reaches the inbox because nothing appears out of place.
From a technical perspective, internal controls are doing what they were designed to do. The weakness sits entirely with the vendor’s email setup, which was never properly examined or monitored.
Why looking only inward is not enough
Most organisations monitor their own email domains closely. Very few look outward at the domains they trust the most.
Standard DMARC reporting tells you whether your systems are sending email correctly. It does not tell you when a trusted external sender starts having problems.
Vendor email environments change constantly. Mail platforms are migrated. New sending tools are added. Keys expire. SPF records grow until they quietly stop working.
When a trusted partner’s SPF breaks, their legitimate emails may start landing in junk folders or not arrive at all. If that sender is a law firm or a financial institution, the impact is not limited to inconvenience. Missed filings, delayed approvals, or failed closing communications quickly turn an email issue into a business continuity problem.
From the recipient’s point of view, the sender still looks legitimate. The failure is silent, and the consequences are usually noticed too late.
Where problems usually start
When organisations eventually review vendor email security, the same issues appear repeatedly.
Vendors change something in their email infrastructure and break authentication without realising it. Everything appears fine until that weakness is exploited.
Many vendors leave DMARC in monitoring mode indefinitely. There is a common belief, especially outside security teams, that monitoring provides some level of protection. It does not. A DMARC policy set to p=none offers no defence against spoofing. It only reports that spoofing is happening.
Short-term subdomains created for campaigns or integrations are often forgotten once the project ends. They remain active, trusted, and unchecked, making them easy to misuse.
These are not advanced attack techniques. They are predictable outcomes of limited oversight.
Why questionnaires do not solve this
Vendor risk is often managed through annual reviews and questionnaires. A vendor confirms that they use DMARC, and the process moves on.
Email security does not remain stable for a year. A single DNS change can weaken controls overnight. By the time the next review takes place, the exposure may already have been used.
This is why vendor email risk needs ongoing visibility. Monitoring the email security posture of critical partners makes it possible to see when something changes, rather than discovering the issue after an incident or a delivery failure.
For high-trust vendors, this visibility matters far more than for low-impact tools.
Moving from assumption to awareness
More mature organisations make a simple shift in how they think about email security.
Instead of focusing only on whether their own domain is secure, they identify which external senders carry the most authority inside the business. Those vendors are tracked deliberately. Their authentication posture is monitored over time, and changes are investigated before they cause disruption.
This allows teams to spot problems early, have informed conversations with partners, and reduce the risk of email being misused through someone else’s weakness.
Monitoring critical vendors does not replace internal email controls. It completes them.
Because when an incident arrives through a trusted external sender, the question is not how well your own domain was configured. It is whether anyone was paying attention to the email security of the partner everyone already trusted.
