TLDR
Even if it's not your issue, you now own the problem. The customer has contacted you guys on this issue before and wasn't given a good enough answer. Now the customer is contacting you guys again in hopes of a better answer. If you don't give a good answer, then the customer may continue contacting you guys. You aren't saving time by brushing the customer off.
You could have saved the day by looking up the information in the first place and relaying that to the customer. If and email isn't doing the trick, then place a phone call.
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I used to do tech support for Microsoft Windows and we gave our best effort to give the customer information to fix dang near anything on their computer. Most people who called in were in our area of support, but sometimes we would get people calling in about other products. People saw us as their last line of support, their only lifeline when nobody else could help them.
I think we were effective in this role because more than any other support desk, we had a great big picture view. A support rep for Intuit might know Quickbooks well, but might know nothing about the rest of the system which Quickbooks sits on. So, in some cases they might say "It's a Windows problem" and pawn the issue on us.
I think just talking to people over the phone really helps as well. When you have someone who is so obviously tech challenged, then it's sometimes just that much more difficult when dealing with yet another tech abstraction over basic communication.
So, customer calls in with something we don't support. In a friendly conversation we would explain that big picture view and tell the customer exactly what they need to know when contacting Quickbooks again. Sometimes the issue was so simple we could just Google the answer. Sure, it's wasting valuable support time, but in the end it could have saved time because the customer wouldn't continue to be bounced back and forth.