Found 2 records.

Status: Verified (2)

RFC 1036, "Standard for interchange of USENET messages", December 1987

Note: This RFC has been obsoleted by RFC 5536, RFC 5537

Errata ID: 561
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Florian Weimer
Date Reported: 2001-05-17

Section 2.1.4 says:

   If the message is submitted in response to another message (e.g.,
   is a follow-up) the default subject should begin with the four
   characters "Re:", and the "References" line is required.

It should say:

   If the message is submitted in response to another message (e.g.,
   is a follow-up) the default subject should begin with the four
   characters "Re: ", and the "References" line is required.

Errata ID: 1760
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Hans Bot
Date Reported: 2009-04-09
Verifier Name: Alexey Melnikov
Date Verified: 2010-09-02

Section 2.2.5 says:

    This command should generate a
    "Subject" line which is the same as the original message, except
    that if the original subject does not begin with "Re:" or "re:", the
    four characters "Re:" are inserted before the subject. 

It should say:

   This command should generate a 
   "Subject" line which is the same as the original message, except 
   that if the original subject does not begin with "Re: " or "re: ",
   the four characters "Re: " are inserted before the subject.

Notes:

Space has been omitted three times. Please compare with original text in RFC850 to confirm that the spaces should be there. (http://www.w3.org/Protocols/rfc850/rfc850.html)

Alexey: also see RFC 5322.

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