The key here is to have an implementation schedule crafted so there is a massive differential impact on a selected group of small providers and a negligible impact on overall performance. After issuing warnings for a few months, put a cap on the number of plain-text submissions per domain, large enough to affect just the worldwide top 5-10 domains in that group. As more domains comply, roll that window, so as to keep just a very small fraction of total mail volume delayed, both incoming and outgoing. Since mail delays are common, the risk of switching by your own customers is low, and special exceptions can be made for accounts that communicate with an affected domain often.
It's essential that the major email domains work together so that the users of small non-encrypting domains feel a generalized performance impact, not just a problem with Gmail or just Yahoo.
It's essential that the major email domains work together so that the users of small non-encrypting domains feel a generalized performance impact, not just a problem with Gmail or just Yahoo.