You will have notified your participants of their matches, and sent them information about what to do for speed mentoring.

You could, additionally, send a reminder email a few days or weeks later, to prompt mentees and mentors to contact each other for speed mentoring. This might be helpful because participants might not have seen the original email, they might have misplaced it, or might have simply forgotten to do so.

You could use a slightly adapted version of your original email as a prompt.