Whitelisting the rctrader.com domain or email address will make sure that emails from us will get through to you, including emails from buyers via the RCT email system.
Depending on your email service provider, there are may different ways to whitelist the rctrader.com domain (also known as adding to safe-sender list).
Rather than go through all of that here, there are a few good resources that have already gathered all the information you need depending on your email provider.
The domain to whitelist:
Specific email addresses to whitelist: