Don’t respond negatively. Take a breath, eat some biscuits, delete the e-mail.
Don’t write back asking why not.
Don’t write back asking for a referral.
Don’t be surprised when you get rejections.
Don’t be discouraged.
Don’t bash the agent, especially not online. Sharing your experience with your peers in an appropriate manner is not the same as bashing.
Don’t take it personally.
Don’t stop writing.
Don’t stop querying.