When you need to respond to a message or email that you've received, it can be helpful to use synonyms for "write back" to keep your language varied and fresh. Some options include "reply," "respond," "answer," "acknowledge," "return," and "comment." You may also choose to use phrases like "get back to," "touch base," or "follow up with." Depending on the situation, you may want to convey a tone of gratitude, urgency, or formality through your language choice. Whatever synonym you choose, the most important thing is to ensure that your response is clear and effective in communicating your intended message.