Internet-Draft expires April 2022
Billon & Levine Expires 16 October 2022 [Page]
Network Working Group
Intended Status:
Standards Track
B. Billon
J. Levine
Standcore LLC

Updated Use of the Expires Message Header Field


This document allows broader use of the Expires message header field for SMTP. Senders can then indicate when a message sent becomes valueless and can safely be deleted, while recipients would use the information to delete these valueless messages.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 16 October 2022.

Table of Contents

1. Introduction

The date and time of expiration can be used by the mailbox provider or the MUA to indicate to the user that certain messages could be deleted, in an attempt to unclutter the user's mailbox and spare storage resources.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2. Header Field example

The field definition and syntax remain the same.

expires = "Expires" ":" date-time


Expires: Wed, 1 Dec 2021 17:22:57 +0000

Senders MUST NOT include more than one Expires header in the message they send.

If there is more than one Expires header then receivers SHOULD treat this as if no Expires header is present.

3. Security considerations

Dates in this header can be set a long way in the past or in the future, including outside the range of internal time representations in some programming environments - all software which processes the Expires header MUST be made safe against this possibility.

4. Advice to Senders

Senders SHOULD add the header field along with a relevant date and time when they know that the content of the message has no value after a given point of time (e.g. Commercial newsletters --especially when including time-limited offers, Event announcements, Social notifications, Time-limited access codes ...).

In all other cases, senders SHOULD NOT set an Expires header.

5. Advice to Receivers (Mailbox providers, Webmails and MUAs)

The expiration of a message's validity would logically lead to the deletion of the message. However, users on most systems do not expect their emails to disappear, and may not be aware that any particular email has an Expires header. Therefore, no email should be silently and automatically deleted solely based on the value of the Expires header field.

Mailbox providers SHOULD explain to users how the information provided in the Expires header are processed, SHOULD indicate when viewing an expired message, and SHOULD give users control over the actions to take for expired messages.

The information provided in the header should be used as a signal that could be used to provide a feature or improved experience to the end-user. For instance, systems may allow users to set up an automatic rule to clean up expired email from specific senders or with specific characteristics, or provide a mode to quickly view and process all expired email.

In certain cases, email messages can be used as proof or element of investigation. As an early deletion may compromise the intended investigation, mailbox providers can ignore the Expires information in such cases.

Presence of the Expires header field MUST NOT be interpreted as a sign of legitimacy.

6. Past History of the Expires: header

[RFC4021] defines a number of header fields that can be added to Internet messages such as those used for mapping between X.400 and RFC822/MIME [RFC2156]. One of them is the Expires header field that provides the date and time at which a message is considered to lose its validity.

The same principle can be applied to the Expires header field in a SMTP context, whether the message comes from a X.400 gateway as initially intended in [RFC2156], or from a RFC821/SMTP MTA.

7. Acknowledgements

This document was informed by discussions with and/or contributions from Jonathan Loriaux, Charles Sauthier and Simon Bressier.

8. IANA Considerations

IANA is requested to update an existing entry in the Permanent Message Headers Field Names registry

Header field name: Expires

Applicable protocol: mail

Status: standard

Author/Change controller: IETF

Specification document: this document

9. Normative References

Kille, S., "MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME", RFC 2156, DOI 10.17487/RFC2156, , <>.

10. Informative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Klyne, G. and J. Palme, "Registration of Mail and MIME Header Fields", RFC 4021, DOI 10.17487/RFC4021, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

Authors' Addresses

Benjamin Billon
John Levine
Standcore LLC