[Mailman-Users] Undeliverable mail

[Elvis Fernandes]

When I send an email to the mail list unix_sysadmin, I get the following
msg:

Your message was not delivered to the following recipients:

unix_sysadmin at utc80.am.com: <unix_sysadmin at utc80.am.necel.com:> 550
5.7.1 <unix_sysadmin at utc80.am.com>
<unix_sysadmin at utc80.am.necel.com>... Relaying denied

Howevre, when I login to the above mailman server box "utc80" and send
an email to unix_sysadmin, I do receive the email.

Could someone tell me what is going on, and how I could fix it?

Thanks
Elvis Fernandes