Could you please clarify, how are you adding emails as tasks? Are you using the Chrome extensions, outlook plugin...etc? Are you trying to open them with Gmail?
What OS and version are you running?
What happens when you click the link in the task created from an email? Does it only happen on one plugin/app? Or all of them that you are using?
Also, have you moved the email after adding the task? Please refer to this topic. http://todoist.com/Support/showQuestion/418/ If you are using the Outlook plugin, please try to install this version.
I am using the Outlook plugin on a University Exchange Server. I reviewed previous users' issues with this and it seems to me that it doesn't really matter whether or not I have moved the email. When I click on it, it simply does not open even if it is still in my inbox. My primary reason for getting todoist and upgrading was to be able to have one simple task list that inlcuded emails. I also hoped to delete the emails from my inbox once i added them to my task list. Should I reinstall the version above?
We are aware of an issue when moving an email with Outlook, and aware that it does not always require it to be moved. The linked version could fix your issue. It's worth a try.
Any more information about how you manage your emails could be useful in finding a permanent solution.
For now, you could try to use the alternative way of managing your mails with Todoist - sending mails to your projects. This won't create links to your messages, but will make the complete message accessible from anywhere as it will convert its body to a task note. This also solves the issue when wanting to delete the emails from your inbox.
Please refer to our blog post comparing both options: http://todoist.com/blog/2013/01/zero-inbox-policy-with-todoist/