Adam and Alex Lambert, Messrs.
Basically, email replies usually follow the normal pattern of writing professional emails.
You also confirm that nothing in your comment infringes third party rights for example, the use of a text from a third party without his or her permission.
It's not wrong to use Dear in your email, but it can come off as too formal especially if you go with the full dear sir or madam.
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There is no right or wrong with an informal letter.