The Tax Mailbox is a formal channel of communication with the SAT, not a secondary information inbox. Through it, acts, resolutions and requests may be served whose deadlines run even if the message is attended to late. In 2026 there is an extension for applying the penalty for not enabling it, but the obligation remains in force. Companies need to combine technical configuration, backup owners and a response protocol that preserves acknowledgments, documents and critical dates.
Who must enable it in 2026
Article 17-K of the Federal Tax Code assigns a Tax Mailbox to persons registered in the RFC. The SAT states that, in general, it must be enabled except in cases of exception or option, such as certain individuals with no obligations, no activity or suspended, suspended legal entities and cancelled taxpayers. Those facilities may cease to apply when a procedure requires the mailbox or when there are recent CFDIs in the cases indicated by the authority. For active legal entities, the practical rule is to treat it as mandatory. The 2026 Miscellaneous Tax Resolution defers to January 1, 2027 the application of the penalty for not enabling it or not updating the contacts; it does not eliminate the obligation during 2026.
The notice can take effect without opening it
The SAT sends an alert to the registered contact methods when it deposits a document. If the taxpayer opens it within the following three days, the notice is deemed made at that time; if it does not open it, it may be deemed effective on the fourth day under the applicable rules. That is why relying on a single person or an abandoned generic address is risky. Email and mobile phone must be registered and confirmed, the acknowledgments kept and the contact methods validated when staff change. In addition, the official calendar matters: from July 20 to 31, 2026 the SAT reported non-working days for certain purposes, so each deadline must be calculated from its specific basis.
A corporate protocol of review and escalation
The recommended control includes daily review on business days, restricted access with a backup, a log of each entry and immediate download of the document and its acknowledgment. The operator must not decide alone on the relevance of the message: they must classify it and escalate it to tax, accounting, legal or management with an internal deadline earlier than the legal one. It is also advisable to separate genuine alerts from fraud attempts; the email or SMS only alerts, while the document is consulted within the official portal. Each file must keep a screenshot or acknowledgment, the opening date, the owner, the response sent and confirmation of receipt. In this way, the mailbox is integrated into the control system and does not depend on individual memory.
Key points
- The extension of penalties to 2027 does not suspend the obligation to enable and keep the mailbox updated.
- A notice may be deemed made even if the taxpayer does not open the document.
- Active companies should operate on the presumption that it is mandatory, unless a specific analysis says otherwise.
- Email and SMS are alerts; the formal act is consulted and attended to within the official portal.
What to review
- Confirm the email, phone, current access and enablement acknowledgments of the Tax Mailbox.
- Appoint a holder and a backup with daily review, a log and escalation by type of act.
- Test the protocol quarterly with a drill of receipt, deadline calculation and response.