It's not your responsibility whether or not she chooses to read her e-mails. If you're really anxious about it you can put "Not urgent" or "When you're back from vacation." If it's helpful to you to send the e-mail to get your thoughts out I'd go ahead. If you think you'd end up waiting for a response, then set the expectation with yourself that there will be no response and that you'll follow up on it in session. Don't deny yourself an outlet, unless your therapist has set that limit. If you do have urgent issues, however, follow the procedure of using your coping skills or going to the hospital if needed.