SpamAssassin is flagging my e-mails as
HTML_20_30 - 'Message is 20% to 30% HTML'
What exactly does this message mean and how can I make it go away?
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I just sent myself a simple test message and SA flags it will all of these:
SA has hundreds of test that each e-mail goes through, some stick, most don't. Each test has an associated score; some tests are good (like the SPF_PASS above); some are bad (like the EXTRA_MPART_TYPE), and others are neutral (like HTML_20_30).
SA uses the score to determine if it's spam or ham. Just because a test comes back positive doesn't make the e-mail automatically spam. My personal server is set so that scores at or above 4.5 are spam. Most businesses set theirs up around 7 to 10. If an ISP was to use SA, they'd probably put it up even higher.
To directly answer your question: HTML_20_30 means that SA's test for 20 to 30% HTML came back positive, most likely because there are HTML markup tags in your e-mail (which is very common).
Chris has already answered the question of what the message means.
If you want to make it go away, you need to remove the related test from SpamAssassin. This is only possible if you have direct access to the server that does the spam filtering. Under Debian, the rule definition files live by default in /var/lib/spamassassin/ (and subfolders). Grep for the output message or the header (HTML_20_30) to find the file, then comment out the rule (or adjust it).