Are your e-mails not sending out? There is a chance that your e-mails are on a 
blacklist. There are hundreds of blacklists out there. These blacklists have been created by a group of white-hat, “do-gooders” to prevent spam, viruses, and other malicious material to be spread via e-mail addresses. They will send you an automatic e-mail to your attempted e-mail that reads something like: “550 Error message…rejected because your SMTP server, 192.168.0.1 is in the Spamhaus RBL…
If you get an error 550 or something regarding a blacklist, spam, or relay error, then you have been blacklisted. Your personal e-mail address, or company’s e-mail IP address, or Exchange server, might have been blacklisted if:
-Your computer or network is infected with spam
-Your computer or network is infected with viruses
-You send bulk e-mails. Note that many e-mail hosts have a limit of e-mail addresses you can send to per e-mail and per day. If you go over this limit, your e-mail address may be flagged as spamming.
-You use a relay or middle-person to send your e-mail. Some e-mail hosts use 2 IP addresses or ways to send your e-mails out. This may be flagged as the middle-person or relay address as a spammer or hacker to spread malicious material.
So, how do you remove yourself from a blacklist?
-Remove all instances or spam, viruses, and unnecessary open router/firewall ports
-Ensure you are not e-mailing over the maximum capacity your email host allows
-Trace your e-mails to ensure they are not being relayed illegally. You may do this by going to DNS Stuff , sign up as a free account, and then enter your IP address (it will already be listed on the top-right corner of the screen) to check out your e-mail travels. If you see other networks relaying your e-mails, you should contact your email host.
-Request removal from the blacklist(s) by clicking on the link your undelivered e-mail provides you. That will allow you to request removal of your blacklisted e-mail address or IP. You’ll need to take the above steps first though or your request will be denied. An example is being asked to enter your IP address for removal at the following site: http://relays.osirusoft.com/cgi-bin/rbcheck.cgi
Overall, you just need to be very careful to prevent blacklisting of your e-mail or IP addresses or else communication and future operations may be halted.
