It's based on SpamAssassin, a popular spam filter used by many corporate networks.
Even though your subscriber engagement and reputation are the most important factors shaping your deliverability, a high SpamAssassin score (higher than 2 or 3) can still get you blocked at corporate firewalls.
SpamAssassin screens your email for spammy-looking content. The rules and scores it uses are SpamAssassin's own (we have no say over them), but this is how you can fix some of the most frequent warnings:
|Warning||How you can fix it|
|HTML_90_100||Your email has too much HTML code for so little text content (90_100 means a 90% to 100% HTML-to-text ratio). Try to add some text to even things out.|
|HTML_TAG_EXIST_TBODY||Your HTML code contains a "tbody" tag, which is not meant to be used in emails. Simply remove it.|
|SUBJ_ALL_CAPS||Your email subject is in all caps. Try not to do that, as it looks like you're shouting :)|
|HTML_IMAGE_RATIO_02||The images in your email are too large for the amount of text provided. Try to add some text or reduce the image dimensions to even things out.|
|HTML_IMAGE_ONLY_08||You have one or more images in your email but too little text to go with it. Try not to design image-only emails, as those are often seen as spam.|
|FUZZY_CREDIT||SpamAssassin thinks you're trying to hide a spammy word ("credit" in this case) in your email text. This can happen if, say, you have a sentence such as "crediting your source". Just reword it to take out the offending word.|
|URIBL_BLACK||A domain in your email or sender address has been included in the URIBL blacklist. This is quite bad! Make sure to ask URIBL why the domain has been listed and request a de-list.|
|MIME_QP_LONG_LINE||Your email probably contains a link with more than 76 characters. Email links shouldn't be this long, even though most servers accept them.|
SpamAssassin can spew out other warnings that you can check by clicking here :)