I send transactional emails from my application on my customers behalf. I'd like for the receiver to be able to click reply and have the reply go directly to my customer, without ever touching my server.
If I send an email from me@myapp.com with the reply-to header set to customer@gmail.com will my transactional emails be flagged as spam? Is this even the slightest red flag to spam filters? It's important my emails don't get caught in the spam folder and i've spent quite a bit of time ensuring that my email is sent properly so I just want to be sure before I implement something like that. Or is this a fairly common practice that I shouldn't be concerned at all about?
I was able to find the answer at another location http://www.quora.com/Does-sending-emails-with-a-From-address-with-a-different-domain-from-the-Reply-To-address-hurt-ones-deliverability
Apparently the reply-to header can be set to another domain without any threat to the deliverability of the email. On some of the smaller mail admins this may cause a problem, but technically speaking its perfectly legitimate.
Provided your sender address is properly set and matches your DKIM/SPF DNS records, you're correct that a reply-to should not negatively effect your sender reputation.
But ultimately it's a provider level decision, so YMMV across different inbox providers.
来源：https://stackoverflow.com/questions/29112817/will-setting-a-reply-to-address-at-a-different-domain-result-in-spam