Proper spelling, grammar, and punctuation is absolutely required in all communication.
There’s no need to put a capital and a period on every message in Slack, but please use proper grammar and spelling at the very least. It makes understanding your message that much easier.
When writing a new email, please use your signature. There’s no need for your signature on subsequent replies to the email.
When responding to a fresh email from an unfamiliar person, use your signature on the first reply. This ensures that person knows who you are and has your contact information.
The goal is to ensure your contact information is readily available, but not to the point of cluttering communications.
Names of things and comments should be properly spelled and grammatically correct.