On Tue, 2021-11-16 at 11:32 +0100, Philipp Ewald wrote: > This is correct. But why is us-ascii requeired for this rule? Are > spammer only in US? > No, its because the base character set for e-mail bodies is USASCII.
Base64 encoding is a way of making sure that attachments using other charsets (UTF8, and those using 16 bit encoding) will look just like USASCII attachments to mail-handling programs, etc and not cause those programs to have reject the mail message. As far as I know it has no other common, legitimate use, but it does have the side effect of making anything thats base 64-encoded unreadable. So, you can see that the ONLY effect of using base64 encoding on an attachment containing usascii text is to make it unreadable. This is why spammers use it: they've worked out that SA will spot and score malicious URLs, shortners, etc. So, some spammers think that using base64 encoding will hide those bad URLs from SA, which is quite true. However their tiny minds don't see that using base64 encoding on a usascii attachment is a fairly reliable spam indicator all by itself. Martin
