I have the outllok plug in and have converted emails to tasks but it doesn't take me back to the email when I click on the link
How are you trying to open these emails, on a mobile device or somewhere else other than Outlook? This unfortunately, isn't going to be possible. You need to open the email from where you created it, Outlook for Outlook, Gmail for Gmail. If you are trying to open them up within Outlook, did you move the email after creating the task?
This is a known issue, and I suggest uninstalling the current plugin, clear the cache for IE, and install this updated unreleased version. https://d2dq6e731uoz0t.cloudfront.net/27d69392edce0cbd2d7128b9c74d070d/as/OutlookTodoist.Adx.exe