From Varkonyi v. State, 276 S.W.3d 27 (Tex. App. 2008):
[A] letter is properly authenticated under [Texas] Rule [of Evidence] 901(b)(4) if its appearance, contents, substance, or other distinctive characteristics, taken in conjunction with circumstances, support a finding that the document is what its proponent claims. Tex.R.Evid. 901(b ...
From Varkonyi v. State, 276 S.W.3d 27 (Tex. App. 2008):
[A] letter is properly authenticated under [Texas] Rule [of Evidence] 901(b)(4) if its appearance, contents, substance, or other distinctive characteristics, taken in conjunction with circumstances, support a finding that the…