This feature requires Author user level or higher.
Spam filters scan all the links in your campaign, and may alter the way they look for some subscribers.
If it looks like the subscriber's spam filter pushed the redirect link code from our tracking into the content itself, turn tracking off, and send a copy of the campaign to the same subscriber. If it displays better, ask the subscriber to find out from their IT person or team why that's happening.
In other cases, anti-virus software could be intercepting the code. These settings vary from subscriber to subscriber, so encourage anyone seeing this problem to contact their IT person.