How can I configure as Invisible text found in the email. This filter create significant number of false positive since high score value (60), and the email is OK. There are no invisible text.

Dalibor,

Use the following steps to modify the settings for this filter.

• Log in as 'admin'
• Go to Filter Management/Content Filters/Custom Filters
• Change the score for "Invisible Text"

Suggestion: I recommend you contact our support department via email (support@synametrics.com) and send us a sample of an email that you think does not have any invisible text but Xeams is incorrectly marking it. Use instructions on https://www.xeams.com/forward-sample-to-support.htm to submit a sample.

Why 1px font size is clarified as "negative"? It might be design reason (some spacing). How can "invisible" text harm? I think 1px font size and invisible text should be removed.

Specifying very small font size, having white colored text on a white background, and CSS styles that are hidden are all techniques used by spammers to hide data. Typically, their goal is to make the reader look at a URL or an image. Having a very short email only containing a URL is often caught by content filtering. Spammers know that and therefore, try to confuse filters by adding irrelevant content that is invisible.

Try running the report for Custom Filters (under Reports). That will give you a summary of messages that are caught by every rule, allowing you to adjust their scores if required.

It is a good method of antispam,if you don't need,you can just disable it.  No one forege you to use this function.