getting references

I am writing a software product for business use. First few versions were for free, but now I started selling the product. I think that adding at least a short list of companies that are using my product to my website would add credibility. There is quite high probability that some of the biggest companies are using my product - I have support request e-mails in my inbox from addresses belonging to several Big Companies. However, I have no certainty that they actually went with using my software beyond evaluation phase. I tried sending a short survey to get some more insight, but the response ratio was very low - in some cases the people that contacted me no longer work for the company (e-mails bounced).

How would you approach this situation? Ideas that come to mind:

1. Use the BigCo name because if they had a support question, they must have been using it.
2. Try to confirm further, by sending a paper letter to headquaters. (Would BigCo care to answer?)

What do you think about the above two? Any other ideas?

Using a company's logo or showcasing them as a user of your product is a sensitive subject. You should ideally get approval from any big corporation for using their logo or showcasing them as a client.

However, most of these companies never give approval for such use, so most people just use them and remove them if the company requests it.

That said... I would only use a company's logo if they were a paid user of my product, used it regularly and I have had dialog back and forth with them on how they are using it within their organization.

Just because someone with an @bigco.com email address downloaded a trial of your app doesn't really seem enough to showcase them on your website.

Be careful with this, I would try a personal email message not a survey.
Something from you, very personal just checking in to get some feedback on the application. I tend to respond to emails from software products I use that appear to come from a human, but don't fill out surveys.