Requested action not taken: mailbox unavailable
Server replied: 550 Verification failed for <test@domain.com>
Connection rejected: SPAM source domain.com is manually blacklisted.
Sender verify failed
SSH into the server and take a look at this file:
vim /etc/blockeddomains
is your trouble domain in that list? If so, remove it, save the file, the restart exim:
service exim restart
That might work