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Robert McClean
When you are an ESP you need to connect with mail exchange servers all over the world in order to deliver emails. We have found that very few systems will accept IPv6 connections (Google are an exception). This leaves us with dual-stacking which takes extra time, and time is money. When it comes to sending the email, the only way to find out if IPv6 is supported is to attempt to connect using IPv6 then fall back to IPv4 if that is unsuccessful. This slows the process down to the extent that more server capacity is required, which costs money. These are the obstacles we face in adopting IPv6.