Well you need follow:
1. You need some web based script to allow real estate agency create a design for HTML mails, save it in DB i guess with some markers to replace it after with client name and other.
2. Click a button and send.... well to do so you will need as script to send it via cron with some delay + you need some very good email address from where you going to send it. It should not be in some spamhose DB or in other black lists...
3. You need to set SIP record for your domain....
But even all of this cant gave you 100% sure that mail will not go to spam folder, here is not common rules. But idea that source host/address need to have good reputation + customers who suppose to get this mails should not send it to spam folder.
Here many requirements to email text and so on to not get in spam folder (see a Google guide lines for example). Anyway if you chose us (i am not alone, i have a team at this site), we can discuses it more. Bye for now