I saw a while back a comment on the back of the new Retention policy about the risk of someone in a Nurture Program getting sent the first/second/third etc emails as there would be no record in the log of them ever having been sent these emails before via the nurture program - because of the 25 month retention policy. But I didn't see anyone coming up with a solution as to how to ensure this doesn't happen - so I got in touch with Support - here is their reply: -
When we send out an email via Engagement Programs we record what emails was sent to a lead in a table in Marketo. The 25 month data retention policy does not affect the data contained in this table. All emails that was sent to a lead is stored in this specific table for the lead which is why we know if an email was sent to a lead before or not.
I know that my colleague that works with documentation is working to clarify this and let the whole community know as well so there is no ambiguity about this going forward.
Thought you'd like to know...