Maximum font size to avoid spam filters

I have seen 18px and 20px mentioned (e.g. here by BThies) as maximum font sizes for avoiding the wrath of spam filters, but never anywhere other than in comments here and elsewhere. It's not mentioned in the CM documentation and resources as far as I can tell. Has anyone heard this from a well-known, reputable source? (This is not meant to besmirch BThies, but I just don't know where he got that info.) I understand why spam filters would block excessive large text, but those sizes are pretty limiting if the design calls for a big headline. If the trigger is more likely to be a large amount of big text, that would be good to know, too.

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