You have been utilizing the default tracking links to send Emails to leads in Marketo and decide to change the default tracking link to your own branding tracking. When you do so, this breaks the links in emails you have already sent.
The branding tracking link should be one of the first steps that the customer(s) need to do when they obtain their instances. If you decided to change the default tracking link to a branded tracking link after utilizing them in emails, you should be prepared in advance that any email sent prior to the change will generate the 404 Error. We recommend not sending any emails within a week or two of the change so that an email sent on Monday is then broken on Tuesday.