Was my student being disrespectful by using shouting language in her email to me?
For a one-off or short-term rudeness, my policy is to respond with pure facts, served chilled. If you have a good instinct for delivering comebacks at just the right level, a hint (but just a hint) of sarcasm might work wonders.
Manners are important, but it's not our job to teach the students manners - and they are rarely grateful for it, especially those who would need such a lesson the most. Most certainly, I generally ignore capitalized letters as they signal inadequate acquaintance with netiquette or a really immature person.
That being said, your student is probably more in total panic and loss of control of the situation than expressly rude; this probably deserves more of your compassion than your anger. This does not preclude you to use aforementioned strategies for response and to decide how much time you are willing to allocate to help them and to enforce your decisions strictly. How they decide to communicate is their business - understanding why they do that, and what to respond are your businesses; and two separate ones, at that.
Do not use language that can be interpreted as a threat (&quot;she should be careful about what she said&quot;), a better response to a challenge (&quot;you challenge me&quot;) is to just ignore it or to ask - without any emotion -what they mean if you really cannot ignore it. Think of Spock's raised eyebrow when you do it, it will put you in the right attitude for this.