What you need is a preview system, which will run a test email through a number of different email applications.
Most of us send out frequent promotional emails, but have you any idea what they look like in email applications other than the one you use yourself, where you sent the test email? Trust me, not everyone in business uses Microsoft Outlook any more. I have one of the most widely-used email clients in the world, Google's GMail, and it's amazing how many nicely-laid-out email newsletters in my inbox look anything but nicely laid out. If they don't work in GMail, the chances of these emails looking good in the majority of other email applications is very small. And if they don't look good, nor does your company.
What you need is a preview system, which will run a test email through a number of different email applications and show you the results. There are several available, and although they all charge, it's a small price to pay for peace of mind (and possibly vastly increased response). Some mailing systems – such as Mailchimp – have their own built-in preview systems, but there are third-party services which you can use too. At least one of them, EmailOnAcid, has "one-off" pricing, so you don't need to be a regular customer. Or if you really can't stretch to that, at least find some people with different email applications to your own, and take a look at what they see. Be brave – the results may not be pretty.