The implementation of B2B e-invoicing in Belgium with effect from 1 January provides us with some key lessons and practical insights which may be relevant to e-invoicing mandates elsewhere (including in the UK and Ireland).

Challenges surfaced at the intersection of data quality, contractual frameworks, operational design (especially when coordinated alongside other country implementations), and long-held assumptions about how invoicing processes typically work. The following observations summarise a few key lessons from the Belgian transition. For businesses operating in countries expected to follow a similar route (such as the UK and Ireland), these insights may be useful.

Master data is critical for Peppol e-invoicing

Accurate, well-governed master data consistently made implementation easier. While the Peppol network does not validate master data content, it enforces it by rejecting or not delivering invoices. Issues such as rounding, non-standard country codes, incomplete or mismatched addresses triggered validation warnings or downstream processing failures. Another example is the incorrect or missing Participant ID, which results in non-delivery of the e-invoice.

Organisations that understood this and reviewed and cleansed their master data prior to the go-live date experienced significantly less disruption. Those that treated master data as a secondary priority, encountered avoidable failures, such as misrouting and non-delivery.

Vendor evaluation matters

Businesses should choose an e-invoicing provider based on a clear understanding of its invoicing complexity, operating model, and future-state requirements. A recurring theme was the impact of provider selection decisions made without a sufficiently detailed assessment of organisational needs. In several cases, organisations chose e-invoicing providers based on high-level compliance claims (for example, ‘we cover country X, Y, Z’) rather than a clear understanding of their own invoicing complexity, operating model, and future-state requirements.

While these solutions are technically compliant, they did not always support the full range of business scenarios and technical requirements those organisations had. These limitations typically only surfaced during in implementation, at which point remediation options were restricted by time, contractual terms, and the effort required to rework integrations. Organisations that invested upfront in mapping their invoicing landscape by identifying edge cases and validating provider capabilities, experienced far fewer surprises.

Those who treated vendor selection purely as a technical or procurement exercise often faced additional rework and work around solutions. The broader lesson is that e-invoicing providers are not interchangeable utilities. Selecting the right solution requires alignment with the organisation’s operational reality, not just regulatory compliance.

Review business and billing scenarios before implementation

Organisations that reviewed their business and billing scenarios prior to the implementation set themselves up for success. Neglecting to perform this review created unexpected complexities during the implementation.

The move to structured e-invoicing required organisations to revisit long-standing invoice design choices. Legacy invoicing scenarios did not always survive the transition. Issues arose around complex invoice layouts and certain VAT category combinations, particularly where line-level logic was needed, free-text fields were used and specific scenarios such as self-billing were used.

Attachments also created challenges. While Peppol supports embedded attachments, format and size constraints limited how effectively they could be used in practice. As a result, most organisations ultimately adopted a parallel distribution of supporting documents, or looked at external hosting of documents with references embedded in the invoice.

Organisations that proactively informed suppliers and customers about process changes experienced a significantly smoother transition than those that relied on implied understanding. Because Peppol registrations cannot be shared, each mandated taxpayer must be individually onboarded and registered. This required revised onboarding processes and well-defined role and responsibility definitions.

In some cases, the shift to structured e-invoicing prompted a reassessment of the end-to-end operating model. Several organisations used this as an opportunity to simplify or redesign their approach rather than replicate legacy structures.

Technical expertise with document creation is required

E-invoice document generation was rarely the core problem, but it did create challenges, especially in combination with complex business scenarios and system architectures spanning multiple Enterprise Resource Planning systems (ERPs)/data sources. The process often exposed inconsistencies and ambiguities in upstream source data and business logic.

In several cases, extensible markup language (XML) schemas allowed more structural flexibility than underlying business rules. This resulted in XML files that were technically valid but still rejected by trading partners or failed internal validation and control checks. XML correctness is necessary but not sufficient. Ensuring meaningful consistency, across systems and scenarios, proved far more important to achieving a stable, reliable e‑invoicing setup.

Operations readiness as the real determinant of e-invoicing success

Belgian e-invoicing is not complex in principle, but has proved unforgiving in execution. The most significant challenges did not stem from XML specifications or transport protocols. Instead, data discipline, contractual clarity and long-standing edge cases (which were previously dealt with by manual processes) created the issues. A key success factor in all scenarios was proper preparation and a clear implementation plan. Those that succeeded also had defined fallback procedures and established ownership for incident management.

For organisations preparing for future mandates, Belgium offers a clear lesson: technical capability is essential, but sound data foundations matter even more.

Practical support for a smooth e-invoicing transition

Belgium’s experience offers clear lessons for future mandates. If you’d like to explore how these insights apply to your organisation, get in touch with Christian Balk.

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