Does a user have to be a member of "Day2" for domain cutover? If not, please explain in-depth process of how ARS relay functions during the cutover.
対策
The user does not need to be a member of the DayTwo group during cutover. The only difference that DayTwo group membership makes is at the end of the cutover, when adding proxies to the target objects. If the DayTwo option in the Domain Move UI is selected, members will have their PrimarySMTPAddress in the target set to their proxy address for the moved domain.
Regarding how the ARS relay functions during cutover: During the domain cutover, the MX record for the moving domain is changed to point to a Barracuda SMTP relay device (US Instance), which relays the mail through ARS. All "matched" and migrated user and groups will have ARS records created, regardless of their DayOne/DayTwo membership. ARS references these entry records as the mail is relayed through it, and rewrites the RCPT TO: on the message to the target domain smtp address and delivers the message to the target tenant. If the users are DayOne/DayTwo, their displayed address will also be rewritten as needed.