I've been getting messages like these for several days.
This is an automatically generated Delivery Status Notification
Delivery to the following recipient failed permanently:
Technical details of permanent failure:
PERM_FAILURE: SMTP Error (state 9): 554 5.7.1 Service unavailable; Client host [18.104.22.168] blocked using dnsbl.sorbs.net; Spam Received Recently See: http://www.sorbs.net/lookup.shtml?22.214.171.124
The strange thing? I tried to send the message using GMail's web interface. I have to wonder how many GMail users are getting bounce messages because of a badly designed DNSRBL...