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)(4). The Texas Rules of Evidence Handbook identifies another traditional method of authentication permitted by Rule 901 known as the "reply-letter doctrine." Cathy Cochran, Texas Rules of Evidence Handbook, Article IX: Authentication & Identification, at 915-15 (6th ed. 2005). Under this doctrine, a letter received in the due course of mail purportedly in answer to another letter is prima facie genuine and admissible without further proof of authenticity. Id. A reply letter needs no further authentication because it is unlikely that anyone other than the purported writer would know of and respond to the contents of the earlier letter addressed to him. Id. Because the reply-letter doctrine has been applied to telegrams, Judge Cochran reasons that it logically would apply to e-mail communications. Id.

Armendariz testified at trial that he received the e-mail and attached video from Appellant in direct response to an e-mail sent by Armendariz to Appellant inquiring whether Appellant would send him the "horse movie" file. Under the reply letter doctrine, the e-mail is authenticated.

The e-mail is also authenticated under Rule 901(b)(4).... Armendariz expressly testified that he received the e-mail with the attached video and the officers identified the video attached to the e-mail as the same one shown to them in Appellant's home.... Given the distinctive content of the e-mail and the attached video and the circumstances under which it was received by Armendariz, we cannot conclude that the trial court abused its discretion in deciding that the evidence was authenticated under Rule 901(b)(4). See Shea v. State, 167 S.W.3d 98, 105 (Tex.App.--Waco 2005, pet. ref'd) (State, in prosecution for indecency with a child, adequately authenticated six e-mail communications from defendant to victim; victim testified that she came to know defendant's e-mail address because he would call to confirm that she had received an e-mail from him, several messages made reference to defendant's occupation, one message indicated familiarity with victim's locker number, and victim testified that content of e-mail messages was similar to conversations she had had with defendant over the telephone); Massimo v. State, 144 S.W.3d 210, 216 (Tex.App.--Fort Worth 2004, no pet.) (trial court acted within its discretion in trial for harassment by electronic communication in concluding that copy of certain e-mail allegedly sent by defendant to victim was sufficiently authenticated, where e-mail referred to altercation between defendant and victim that occurred shortly before e-mail was sent, victim recognized defendant's e-mail address, victim testified that only defendant and few other people knew about things discussed in e-mail, victim testified that way in which e-mail was written was way in which defendant would communicate, and witness testified that she saw defendant send similar threatening e-mail to victim using same vulgarities).