Concerned that inquiry replies might be landing in junk mail folder

We have a site with a contact form that people can fill out to send us an email. When they do, we send them a reply, from a Gmail account as it happens, with a subject like: “Re: [example.com] contact from ‘John Smith’”. Since the customer’s email has no knowledge that they ever sent us a contact in the first place (because it went through the form on our site) I’m concerned that some overzealous spam filters might interpret this as an unsolicited email and junk it. I do hope that having the user’s right name in the subject line might help smarter filters figure out that it’s legit. Am I being paranoid here, or is it worth taking steps such as signing up for something like Postmark? Also, would it be better from a spam perspective for replies to come from our domain rather than from Gmail?

Thanks for any thoughts or suggestions.