"=20" etc symbols

I set my list to allow plain text only (no hyper text, no attachment, 
etc).  Occasionally, there are mails through the list having symbols like 
"=20", " =", etc in the mail body (within the text).  I used to use a perl 
recipe from this list (or procmail list; sometimes ago) to clean them:

  perl -pe 's/=([0-9a-f][0-9a-f])/sprintf "%c", hex($1)/ige; s/=\n/\n/'

but it's not always effective. They used to appear often from European 
mails, but now some US orignated mail also have it. I wonder what exactly 
are they and is there a better way clean them out?

Zhiliang