This happened before with some ISPs in 2014 (@jsha wrote about it at ISPs Removing Their Customers' Email Encryption | Electronic Frontier Foundation). It's true that it's not always immediately clear whether it was intentional on the ISP's part; one incentive would be if the ISP wanted the ability to monitor, record, or censor outgoing e-mail, and was willing to block encrypted connections in order to do so.
schoen
23
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