When it comes to internal business communications, every business has different spoken and unspoken rules.
In one business, employees may be expected to be formal in all of the internal correspondence. In another business, employees may be expected to communicate casually. It's important to learn about these rules and act accordingly.
I don't think it's a good idea to insist on a casual or formal type of communication if the opposite is being expected. Despite good intentions, I think that may back-fire and leave a negative impression on others. So it's important to match the business' character and adapt to the environment. If people want to be casual with you, be casual!