It doesn't really matter if he or the gmail user initiates the conversation, in my experience. Probably the only reliable way to communicate with gmail users is to send them 5xx response right away when they first contact you, and inform them there about the situation, and that they should check spam and why, tell them to resend, and let the other messages from the same gmail address through (maybe repeat this from time to time so they will not forget to use their e-mail properly and check spam folder). Let them have some of the fucking time waste their provider causes others.

Because it's really infuriating if someone writets to you, you put time to help them with something, and their demented (did I say it's infuriating? :D - also what kind of company can't detect something is a response, if you even quote the original email, it's ridiculous) e-mail provider trashes your response.