I am getting several different messages when I send e-mail>
recipient could not be reached
550 5.7.1 <email address>... Relaying denied
or
----- The following addresses had permanent fatal errors ----- <email address>
(reason: 550 Service unavailable; Client host [ip address] blocked using bl.spamcop.net;
What does this mean?
That has nothing to do with your mail program, and everything to do
with the IP number that you and your ISP use when you are connected to
the Internet.
Spamcop is one of several DNS "block lists" (sometimes called
blacklists), that are used to stop spam. The recipient's ISP is
running spam-blocking software that works like this:
1. You click "send" and the email goes to your ISP's mail server.
2. Your ISP adds a "Received" line to the email and then attempts to
contact the recipient's mail server.
3. That mail server checks the IP number of your ISP's mail server
against a "block list". Your ISP's IP is on the list, so the message
is rejected.
The reason that your ISP is listed on one or more of these block lists
is usually because someone has complained about spam from that IP
number or that range of IP numbers.
One reason that an IP number can be listed is that it is an "open
relay". The first block lists that became widely available a few
years ago concentrated on those because they were widely abused by
spammers. However, since that time, most open relays have been shut
down. Today most spam comes from other sources, such as open proxies
or "trojaned" (hijacked) residential or business computers.
So I wouldn't necessarily trust that part of the rejection message.
Some mail administrators are sloppy about setting up error messages,
and the message might say "open relay" even if the listing has another
cause.
In the case of Spamcop, its developers acknowledge that it is very
"sensitive" and should probably not be used as your only spam filter,
due to the high rate of false positive (legit messages falsely
identified as spam). However, Spamcop listings are very short-lived,
typically 24 or 48 hours if I recall correctly.
For more information about Spamcop and DNS block lists:
http://www.spamcop.net/fom-serve/cache/1.html
MAPS RSS, another DNS block list:
http://work-rss.mail-abuse.org/rss/faq.html