1. Keep it brief, and focus on key points. Know what you want to say before you start typing. Skip filler phrases like "as you know" or "at this point in time." And in general, try not to waste 10 minutes on an email that would take two minutes on a computer.
If you know you need to write a long email, it's okay to send a quick one-liner that says you'll respond as soon as you get back to your desk.
2. Be careful with "reply" and "reply all." It's easier to overlook when you're on a phone. So just double check to make sure the email you're sending is going to the right people. You might want one person on an email chain to see what you're writing, not everyone.
3. Get comfortable with your voice-to-text feature. Most phones can now dictate what you say as you're saying it. And it makes longer emails go a lot faster. You might just need to learn to use it, and get comfortable with speaking the punctuation as you go.
4. Always proofread. It's a lot easier to make typos on your phone. And sometimes auto-correct can completely change what you mean to say. So always proofread your emails carefully. Especially if you're using voice-to-text.