ENCRYPTION BLOCKING IS OCCURRING TODAY AND THE PROPOSED RULES WOULD Not STOP IT [pdf] (s3.amazonaws.com)

TL;DR: Some mobile ISPs are doing a man in the middle on SMTP connections and stripping out STARTTLS. The FCC's proposed rules wouldn't prohibit this (even though they clearly ought to).

"Users are not receiving the open, neutral, and uninterrupted service to which the Commission says they are entitled."