What is a short URL?
A short URL is a link that has been made shorter and still redirect to the required page. Usually, a user can generate short URL thanks to URL shortener tools (the most famous of them being Bitly).
Short URLs are useful as they allow click tracking and branding of the links shared.
Why you shouldn't use short URLs in your emails?
URL shortener are great tools for marketers, but also for spammers and scammers.
Indeed, it allows them to hide the destination page of a link, and to spread malicious links.
This is why most spam-filters are tracking short URLs inside emails and can put your message in junk if they detects a link that has been shortened.
Not only the use of short URL will decrease your deliverability for given emails, but it will also impact your sender reputation, and have disastrous impact for your email deliverability as a whole.
It is common that people sending emails containing short URLs end up in domain or IP blacklists, and got their deliverability really hurted.