Because IMSVA does not accept the task of relaying the mail in such situation, it will NOT send any bounce notification/message to the sender mailbox, according to RFC5321. Rather, the upstream MTA is supposed to send "undeliverable mail" notification message to the sender.

As stated in RFC5321:

3.6.3. Message Submission Servers as Relays

If an SMTP server has accepted the task of relaying the mail and later finds that the destination is incorrect or that the mail cannot be delivered for some other reason, then it MUST construct an "undeliverable mail" notification message and send it to the originator of the undeliverable mail (as indicated by the reverse- path).

For more information about who should send the "undeliverable mail" notification, please refer to RFC5321.