I also use ReservationManager for all my bookings and I have not had anyone's payment request go to a spam folder. Nevertheless, here's what I do: during my communications with the guest, I tell them that I am sending them a payment request via email. I tell them the subject line will say "payment request for HomeAway property XXXXXX". I let them know that it only takes me a few minutes to prepare the request and to upload the rental agreement and if they do not receive the request within the hour to please check their spam folder.
Just let people know that you are sending a payment request within a certain time frame and if they don't receive it to check their spam folder. No need to send additional emails, just include it in your communications.
Hope this helps!