I just discovered that there is a limit in Outlook 2010 and 2007 for how long a hyperlink is allowed to be before it becomes “unclickable” (i.e. it looks like a link, but no browser window will open when clicking it). Without altering the security settings, any link longer than 1023 characters simply wont open unless you copy and paste it yourself. Not really a problem for many of us, but in a recent project my application sent out emails containing verification links with an encrypted query string parameter. This would have caused a lot of confusion (and frustration) amongst the less IT competent target audience.
This is also the case for “user friendly links” with a different display text (Like this one) – if the underlying hyperlink exceeds the limit it still won’t work.