Just from looking at my own mail servers and mail headers, Gmail will happily both send and receive mail over IPv6, and has for years. Same with Outlook. Oddly,

Just from looking at my own mail servers and mail headers, Gmail will happily both send and receive mail over IPv6, and has for years. Same with Outlook. Oddly, despite Apple's push for IPv6 support, iCloud doesn't seem to have an IPv6-capable MX.