Dot-stuffing
The SMTP rule that doubles a dot at the start of any line of message text, so that a line holding a single dot can safely mark the end of the message.
What it means
After DATA, the message ends at CRLF.CRLF, a line containing only a period. To keep a real line of text from being read as that marker, RFC 5321's transparency rule has the client add a period to any line that begins with one, and the server remove it again. A line that happens to start with a dot is therefore transmitted with two.
Software that skips this step, or treats line endings loosely, can cut messages short or let message text be read as SMTP commands. Disagreement between servers about bare line feeds around the end-of-data marker is what SMTP smuggling exploited. Exchange Online rejects messages containing bare line feed characters with 550 5.6.11.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.5.2: Transparencyrfc-editor.org
- Postfix: SMTP Smugglingpostfix.org
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes