Question : dsn=5.0.0, stat=Service unavailable

Hi!
I am having  problem using sendmail on our Linux 9.0 Server. We are using the latest release version.
Here is the problem:
When I send emails through sendmail for yahoo, hotmail, and some other email providers it works fine, BUT when I use it for my verizon account and some others it says:
ctladdr=caldomain> (0/0), delay=00:00:00, xdelay=00:00:00, mailer=esmtp, pri=30346, relay=relay.verizon.net., dsn=5.0.0, stat=Service unavailable
Jan 19 02:02:14 sendmail[13631]: i0J72EG2013629: i0J72EG2013631: DSN: Service unavailable

Can anyone help with this please?
Thank you! :-)

Answer : dsn=5.0.0, stat=Service unavailable

There are several possible causes:

1) There could be a network problem between your system and relay.verizon.com (external routing issue, path down). And you can see if that's the case with 'ping relay.verizon.com'.

2) Your external IP could be black listed at verizon.com (dial-up IP, part of a netblock owned by an ISP with a poor record of stopping spam). And you can usually figure this out by 'telnet relay.verison.com 25' If you get the SMTP welcome and still can't get a message through it may be a black list issue.

3) Your external IP does not resolve to a Fully Qualified Domain Name (FQDN) or it may resolve to an FQDN that's not the same as what your system offers to the remote MTA as its hostname. Some mail servers are configured to block on either or both of those cases.

4) Messages sent to Verizon might not have a valid, resolvable, sender domain. This could happen where you have a registered domain name, but machines on a private network are't set up as members of that domain and the sender address winds up being bogus. That can also happen by an error in configuring an email client.