Your question is a bit vague because Marketo operates a variety of different "servers." They do not publish AAAA records for web properties, so at that level would not be considered to support IPv6.
As for sending mail to an IPv6-only MX, you'd have to test it, but I believe it does work. However, receiving out-of-band bounces does not work, so it's really bad to send to servers from whom you cannot service returned mail (great way to get blacklisted).
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