Why are my links not working? (Broken link)

This can happen when click tracking is enabled in the email variation (it's enabled by default).

Quickmail tracks clicks by masking the links using a unique URL for every email that are sent out. The unique URL (example: http://u13984.qcml2.io/link...) is attached to the prospect ID, sequence ID and conversation ID. The unique URL, when clicked, will trigger a clicked event and redirect to the URL you originally linked. If any of the associated data is altered or deleted, then the tracked link will no longer redirect to the URL you've set. 
So if the link in the email variation was altered after the link has been sent out, then the tracked link will no longer redirect properly.
Same thing will happen if the prospect has been reset or deleted and if the prospect bounced.
Bounced status deletes the conversation history - resulting to a broken link.
To avoid the issue: 
Never delete email variations or change the links in your email once it's already been sent out. 
You can just pause the email variations you no longer want to use.

Tip: It's best to test the emails using test prospects before starting the sequence on live prospects. 
There is also an option to disable click tracking when editing/creating email variations - so that links will no longer get masked.
Disabling click tracking also improves email deliverability.