If they're running a spam filter on their servers, they would need an unencrypted copy to make decisions on that data. Even if a system existed to filter spam b

If they're running a spam filter on their servers, they would need an unencrypted copy to make decisions on that data. Even if a system existed to filter spam based on encrypted contents without decrypting them, that would allow them to determine the contents of the encrypted message. (see also; Oracle database's data redaction feature) Content-based spam filtering is likely done client-side, and likely less effective than centralized filtration.