Updated Use of the Expires Message Header Field

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Table of Contents

1. Introduction

[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.¶

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

Example:¶

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

There should be only one occurrence of the header field in a single message. The presence of more than one Expires header would make them void.¶

3. Security considerations

The expiration of a message's validity should lead to the deletion of the message. In certain cases, such as emails being used as proof or element of investigation, an early deletion may compromise the intended investigation. For this reason, we want to avoid the header field to be tempered with.¶

4. Implementation and Usage Consideration

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

Generally, no email should be automatically deleted solely based on the value of the Expires header field.¶

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. Automation of email deletion based on the value of the Expires header may be set by the end-user.¶

Receivers can prevent deletion from happening if necessary.¶

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

6. Acknowledgements

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

7. IANA Considerations

This document has no IANA actions.¶

Authors' Addresses