Meaning of TO_NO_BRKTS_HTML_IMG in SpamAssassin

email, spamassassin

Solution

Also ran into this problem and digged in the source code of spamassassin and found this:

meta       __TO_NO_BRKTS_HTML_IMG  __TO_NO_ARROWS_R && !__TO_UNDISCLOSED && HTML_MESSAGE && __ONE_IMG
meta       TO_NO_BRKTS_HTML_IMG    __TO_NO_BRKTS_HTML_IMG && !__FM_TO_ALL_NUMS && !__FROM_FULL_NAME && !__HAS_THREAD_INDEX && !__DKIM_EXISTS && !__HAS_SENDER && !__THREADED && !__LONGLINE 
describe   TO_NO_BRKTS_HTML_IMG    To: lacks brackets and HTML and one image
score      TO_NO_BRKTS_HTML_IMG    2.000   # limit
tflags     TO_NO_BRKTS_HTML_IMG    publish

So, a single image in a email and a missing name in the `to` field seems to trigger this. To be more concrete, this happens, when:

- The recipient does not contain `<` and `>` (`__TO_NO_ARROWS_R`)

- The recipient is undisclosed (`__TO_UNDISCLOSED`)

- The email contains html (`HTML_MESSAGE`)

- The email contains exactly one image (`__ONE_IMG`)

- The sender (from) does not only contain numbers (`__FM_TO_ALL_NUMS`)

- The recipient (to) does not contain a name (eg. `foo@bar.de <foo bar>`) (`__FROM_FULL_NAME`)

- Dont know what this is for, the comment in the sourcecode itself says `# Explain later. ;)` (`__HAS_THREAD_INDEX`)

- No DKIM signature exists (`__DKIM_EXISTS`)

- No `Sender`-header is given (`__HAS_SENDER`)

- Is not part of an conversation (?) (`__THREADED`)

- Line length does not exceed 998 characters regarding to RFC 5322 (`__LONGLINE`)

Problem

I am trying to find out what is the meaning of the flag `TO_NO_BRKTS_HTML_IMG` in SpamAssassin. The provided description for it says: To: lacks brackets and HTML and one image As far as I understand it means the mail message is in HTML format and contains only one image, but what does "To: lacks brackets" mean?

Original source