Are you transferring ownership via SFDC or Marketo? Is the email being sent via Sales Insight or a Marketo campaign?
Whoever has ownership of the Lead should have their information populated when the email is sent if using tokens. But, if you are switching ownership via SFDC, then send an email via Marketo campaigns, the change in ownership probably had not been synced. We have slowed our sync down to every 30-35 minutes.
To my knowledge, is there no way to prevent, or add a wait step, to emails sent from Sales Insight.