Recently i received a complain from our customer said that he cannot send email using webmail and mail client. I suspect his email server IP Address most probably blacklisted by the Spamhaus Block List (“SBL”) Advisory (database of IP addresses from which Spamhaus does not recommend the acceptance of electronic mail).
The main reason why their IP been blacklisted by SBL is because their IP Address been identified as a zombie or intentionally send out SPAM.
Here i going share with you a good website that i use to check the IP reputation. The website called mxtoolbox.
Once u access the website, you just need to click on Blacklists link and enter your IP Address to check on the reputation. If you IP Address been blacklisted, it will be listed. You can just click the blacklisted link to white list your IP. But please take note that If your IP Address still sending out SPAM, you will be blocked again very quickly.