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When submitting a new book to an agent from whom we got feedback in the past, should we address them with Ms./Mr. or first name?
Dear Address Unknown,
When querying an agent for the first time, the general rule is to use Mr. or Ms. with the agent's last name. If you're not sure of the agent's gender, it's fine to use their first name. I'd advise against anything like 'Dear Agent' or 'To Whom it May Concern,' because those greetings make it look like you haven't done your research.
As to your question, you've already gotten feedback from this agent, so I'd look at the earlier email to see how the agent identified themselves. Most likely, they signed the email with their first name. If that's the case, then it's fine to open your next email by addressing them with their first name (since they opened the door to that relative informality). If they didn't sign the email, or if they signed it using something other than their first name, then I'd stick with Mr./Ms. [Agent Last Name] to be on the safe side. Either way, as long as you use either their first name or Mr./Ms. [Agent Last Name], and not something generic like 'Dear Agent,' you should be fine!