Despite what one would think/hope, sending a message to a given address does not inherently give Google a high confidence that a reply from this address is expected (and, for example, that it should bypass spam checks). I have confirmed with Google's tech support that there is no way to automatically have this happen. The user can do the following: 1. Add the address to your contacts list in Gmail. 2. Check spam folder for replies, and mark it as "not spam" if something ends up there, which should influence the fate of future replies received. I can also approve an address at the domain level, i.e. if it is a big vendor or similar. I've had to do this with several of our Chinese vendors. I regularly ask engineering and purchasing to give me a list of the supplies we deal with, so I can approve them as a preventative measure. For what it's worth, all of the false positive instances of reply -> spam we have experienced have involved the sender's email server
The personal blog of Scott McGrath: musician, gadget, and car guy.