It may be a bad alias entry.
This appears to have been initiated by a cron job (which will send one message for every run...) Then you get the error (being sent to root on the local host, which appears to be translated to firstname.lastname@example.org
). If this server has a bad alias entry, then it will attempt to relay.. but if malformed it may not be able to send (or has been denied connection to) a server.
And the mail system will resend the message after each attempt at a connection (which bounces).
The server that is denying a connection is server.hsd1.ma.comcast.net. It is apparently blocking a sendmail connection (either because port 25 is not open, or a firewall rule, or a hosts.deny/allow restriction, but it will be on server.hd1...).
Now, because it appears to be the same as the local host (not absolutely sure on that) this could be happening because sendmail cannot identify itself as "server.hsd1.ma.comcast.net". This can occur if the FQDN is missing from the local host table. This can be overridden in the sendmail.cf file. This is the part:
# my official domain name
# ... define this only if sendmail cannot automatically determine your domain
Sendmail attempts to define the host name by combining names with the domain name... but sometimes it doesn't work. Usually it is because the FQDN is missing from the local host table, sometime because the hostname is not the same as the network name (there is no required correlation).
I'm sure stevea will note any errors I have made... He usually does.