Mass email tracking
In addition to pixel tracking, a second way to track open rates is by looking for clickthroughs. If someone clicked through, then they must have opened it. This is infrequent, but it's important not to throw this data away.
More details:
- How MailChimp tracks open rates
- How CampaignMonitor tracks open rates
- Wikipedia on email open rates
- Hubspot on open rate issues
Yes, this is pretty much the only way to do it. Consider that an email is something that is inherently static. The only way to know if someone has "opened" an email is for the email to send some information back to your server. Most email clients these days support HTML emails, which means that you can get the client to request an image (or anything else) from your server by embedding the proper HTML tags. Other than this, you cannot force an email client to do anything it doesn't want to do. It's a separate program on a remote computer, and you have no control over it.
No, there's no foolproof way. There will always be emails you can't track. If someone downloads their email and disconnects from the internet before reading it, you can't track that email. Most email clients allow you to disable image loading now as well if you want to, so that can block tracking too.
I've usually written my own, so I wouldn't know what to recommend. I imagine most services will be quite similar, so I'd base a product/purchase decision on how easy their front-end is to use.