What aspect of mail service increases the validation of a subpoena’s delivery?

The aspect of mail service that increases the validation of a subpoena’s delivery is the return receipt requirement. When a subpoena is sent via mail with a return receipt requested, it provides tangible proof that the subpoena was received by the intended recipient. The return receipt can be used as evidence in court, showing the date of delivery and the identity of the person who signed for it, which can help to establish that proper notice was given to the witness. This form of mailing creates a reliable record of service, thus enhancing its validity.

In contrast, while the method of mailing used can play a role in how the subpoena is sent, it does not provide the same level of confirmation as a return receipt. The time of mailing may be relevant in certain contexts, but it does not inherently validate the delivery of the subpoena. The witness's previous appearance history can be informative, but it does not affect the validation of how the subpoena itself was delivered. Therefore, the return receipt requirement stands out as the key factor that enhances the credibility and legal standing of the delivery process for a subpoena.