hi, Barney, how are you today?

False Positives

How is your open rate measured? Usually there'll be an image in your email which, when loaded by a unique email address, will count as a single 'open' of the email. If that's what you're doing then I wouldn't be overly worried about 'false positive' opens. Open rate is only ever a guide of success anyway - a reader might open your email for 2 seconds & then delete it, or they may read it word for word. Each of those will count as one 'open', but they're obviously worlds apart in terms of their value to you.

Is there a widely accepted average rate that we should be striving to achieve?

No - there are too many variables. How good are your emails? How much value do they add? How important is your offering to your customers? How time-rich are your readers? How often are you emailing? How good was your last email? How strong is your brand? etc. I've run campaigns that have consistently got 60%+ open rates, equally I've sent emails that have got <5% open rates.

Having said all of that, here are some benchmark figures from MailChimp:

http://www.mailchimp.com/resources/email_marketing_benchmarks.phtml

Watch Out For Short-Termism

Another little note - be careful not to focus solely on your short-term open rates: if you're constantly overselling in your subject lines (to get readers to open the emails), you may damage their trust in you longer-term. Look at your open rates in conjuction with click-through (to make sure you're not overselling in the subject) &, even better, put the emphasis on conversion (or on something like RFV if your emails are purely used to keep customers warm). On the topic of overselling in subject lines - here's an article on writing decent email subjects:

http://www.omstrategy.com/27/how-to-write-a-good-email-newsletter-subject-line

Hope that's of some use!

danie