This is a list of commonly and uncommonly used abbreviations that are used in the subject box of an English-language email header.
Standard prefixes These prefixes are usually automatically inserted by the email client.
Re: or RE: followed by the subject line of a previous message indicates a reply to that message. "Re" in a narrower sense though is, as RFC 5322 3.6.5. explicitly states, an abbreviation of "in re"—"re" being the ablative singular of rēs ("thing", "circumstance")—, loosely meaning "about", "concerning", "regarding". As such, regarding is a fitting English translation with the same two initial letters as in reply. It is expressly stated in RFC 5322 3.6.5. as somewhat structuring the otherwise free-form subject field. If used, exactly one character string Re: (disregarding letter case) ought to appear at the very front of the subject line. Fw:, FW: or FWD: signals a forwarded message: the recipient is informed that the email was originally sent to someone else who has in turn sent a copy of the email to them. The common appearance of "Re:" in email clients has led to the erroneous inference that re requires a colon even in running text.
Non-standard infixes and suffixes These words are inserted in the middle of or at the end of the subject, usually by the author.
Was:, WAS: or was: indicates the subject was changed since the previous email. Not an abbreviation, but the English word "was" (past tense of "to be"). Denoting a subject change prevents confusion on the part of the recipient and avoids accusations of threadjacking in email-based discussion threads. Original subject may furthermore get parenthesised. Example: Subject: Do you know a good babysitter? (WAS: What should we do this weekend?); real-world occurrence: lore.kernel.org OT: off topic. Used within an email thread to indicate that this particular reply is about a different topic than the rest of the thread, in order to avoid accusations of threadjacking. EOM, Eom or eom – end of message. Used at the end of the subject when the entire content of the email is contained in the subject and the body remains empty. This saves the recipient's time because they then do not have to open the message. 1L – One Liner. Used at the beginning of the subject when the subject of the email is the only text contained in the email. This prefix indicates to the reader that it is not necessary to open the email. E.g., "1L: WFH today" WFH – work from home. Used in the subject line or body of the email. NONB – Non-business. Used at the beginning of the subject when the subject of the email is not related to business. This prefix indicates to the reader that the email is not about a work related or endorsed topic.
Software development The following prefixes are often used in software development:
[ANNOUNCE], [ANN] – announcement. A new version of the software has been released. [BUG] – bug report. A description of an error in the software. [PATCH] – software patch. New code is attached to or included in the body of the message.
Other English abbreviations
This is a list of abbreviations which are less commonly used in the subject of an English email header:
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