In this age of dashed off emails and quick texts, many people have a relaxed attitude about grammar in electronic correspondence. Still, your choices in writing make an impression and it’s important to know who you’re writing to and what impression you’re making.
I have a brilliant friend who chooses not to use capitalizations in his email messages. I already know he’s brilliant and I suspect this is a quirky stylistic choice. In casual exchanges, I don’t think twice about that choice. However, if we were working on a book proposal together and his draft of the email to the editor accompanying the proposal didn’t follow the accepted rules of grammar, we’d have a conversation about it. That email is an opportunity to make an impression. Poor grammar is an indication that the writer isn’t detail oriented or that they don’t have much respect for the written word or the reader. Good grammar isn't unfriendly, but poor grammar is unprofessional. Unless you’re writing to very close friends or family, err on the side of correct grammar. It wouldn’t hurt to properly punctuate emails to your bestie, either. Share the grammar love.
Also, remember that part of the work of grammar is to clarify meaning. Your casual approach in email might create more than an impression; it might also lead to misunderstandings and confusion.