It seems that something is generating emails to www-data address without domain:
from=<
[email protected]> to=<www-data>
Mar 16 02:17:06 domain plesk-sendmail[3836147]: S3836146: add-from: stderr: PASS
Mar 16 02:17:06 domain plesk-sendmail[3836147]: S3836146: py-limit-out: stderr...