Spam handling is all about weights. Attachment? I won't block you for that, but I'll give you a point. Valid domain? I won't accept you for that, but I'll subtract a half point.

Then at the end you throw the message away if the sum total of the points passes some threshold "X"

Counting digits in an envelope sender would just be one more metric.

I do some blacklisting, but 99% of the time blacklisting on one fuzzy metric is seen as extremely bad practice in mail.

That's why they said it's only one of many signals one should look for, and not to block transactions on that symptom alone.