Often the most famous generic URL shorteners get blacklisted. A URL blacklist is when a site gets added to a list of unsafe URLs, often resulting in visitors being unable to access the website. When you send an email containing links shortened using a blacklisted URL shortener, your email is likely to end up in spam folders.
By using a branded domain in Rebrandly, your unique domain will be much less likely to be blacklisted or marked as spam.