UpdateThe authors of the plugin suggested that it is a false positive. I am using MailPoet in a Wordpress installation to send newsletters.In an effort to validate the chances a newsletter has to be characterized as 'spam', I sent an email to the 'Newsletters spam test by mail-tester.com'.There, I see an entry with a huge penalty:-1.499        HTML_FONT_TINY      HTML_FONT_TINYThe links to the relevant spamassassin pages do not provide any information.The same goes for the spamassassin tests page.Googling it a bit, seems like it is reffering to a small font size. However, the smaller font size I have set is 14px. This is not a size one would call 'tiny'.How could I fix this?I have also posted a report to the relevant plugin pages.
Spamassasin reports HTML_FONT_TINY with huge penalty, what is it?
html;spam prevention;newsletter;mail
As the author of the plugin suggested, this can be treated as a false-positive.Our SpamAssassin checks on mail-tester trigger a lot of  false-positives, please ignore them.