WU Spam Emails Generally Sent Regarding the Same Subscription Analysis

Encoding tables can be found online, e.g., https://www.ascii-code.com.

1-Proof-of-Work and Emails. Suppose you are tasked to
review a new proposal to counter spam emails.
Whenever a sender wishes to deliver a mail to the
receiving SMTP server, the sender has to include a
nonce n such that SHA-256(recipient email || subject
|| n) < 2 240 . The intuition behind that system is that the sender must have invested some work to find the nonce tailored to each recipient and subject. Thus, mass delivery of spam emails would be very costly. Is this an effective countermeasure against such an adversary? Justify your answer. If it is not effective, how could it be improved?