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I wonder why most email clients prepend Re: to the subject if the mail is a response.

When responding, the mail client generates an In-Reply-To: header which clearly states that this message is a response. This enables mail user agents to display the mails in the correct thread (together with the References: header). So technically the Re: does not add any information.

The disadvantages are:

  • The Re: string needs to be adjusted to the language (Sv:, Odp:, Vs:, etc.) which makes it very hard to configure the mail client properly.
  • It adds visual clutter to the subject line.
  • It can lead to subject lines like this: Re: Re: Sv: Odp: Re: This is the subject

Why is this common practice and what are the advantages?

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closed as off topic by Stéphane Chazelas, Thor, warl0ck, rahmu, uther Feb 6 '13 at 13:37

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That's off-topic here. The in-reply-to and references headers are optional, and different mailers have different ways to thread emails, with Microsoft having been know to use its own silly specific way. – Stéphane Chazelas Feb 6 '13 at 10:58
I assume the use of Re: predates the widespread use of Microsofts mail clients. – Marco Feb 6 '13 at 11:11
The all too common practice of localizing the Re: prefix is actually an error and a violation of the RFC. Arguably the spec can be considered broken, but that does not mean it's okay to violate it. – tripleee Feb 6 '13 at 12:52
@tripleee Are you sure? “MAY start with the string "Re:"” does not imply “May not start with anything else than "Re"”. Which paragraph forbids the localisation of Re:? – Marco Feb 6 '13 at 13:04

The practice originated with paper mails, not emails. In computer terms it's ancient; it's been defined in the RFCs for email messages since at least RFC822. (Now I think of it, it's probably defined in the standards for UUCP and NNTP messages too.)

The current standard for message formats is RFC 5322, which in appendix A.2. states When sending replies, the Subject field is often retained, though prepended with "Re: " as described in section 3.6.5.

It's interesting to note that the "Re" does not mean "reply", but "reference", "regarding", or the Latin version "in re", so most of the translations are wrong. I personally find the translations unnecessary and ugly for precisely the reasons you state.

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Note that RFC 5322 specifically states in section 3.6.5 that the informational fields (of which Subject is one) is unstructured as well as optional. Replies MAY (not shall) have a subject starting with Re: . – Michael Kjörling Feb 6 '13 at 10:58

One reason why this is common would be that many (most common?) e-mail clients on Windows don't do proper threading based on Message-ID:, References: and In-Reply-To:.

Proper threading also requires that the previous message in the thread is available; depending on the mail storage design, it may even need to be available in the same mailbox. On mailing lists (reflectors) this is a minor problem; for one-on-one conversations between individuals, it's a game-changer.

Having a visual cue in the subject line allows the user to see easily, at a glance, from a single entry in the list of emails, whether a message is an original message, a forward, a follow-up or reply, etc.

Forwarded messages commonly don't include any reference information; the MUA would have to go hunting through any mail message-parts, assuming that the mail is even forwarded as a MIME attachment and not simply copied by the MUA into the body of the new email.

Changing convention is a hard thing to do.

All of the above seem like good reasons to keep the tags in the subject line even if technically they could be omitted in some cases.

That said, I agree with you on the translation of those tags, which can create a mess. Not only can the recipient be unfamiliar with the tag the other person's software uses (I'd have no idea what İLT means, for example, let alone תגובה which I doubt I'd even recognize), but some combinations are outright ambiguous (Danish/Norwegian VS: and Finnish VS:, for example), greatly reducing the value of the subject line tag. Wikipedia: List of email subject abbreviations in languages other than English.

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