Case 1: Not Receiving.
On rare occasions, you might not receive mail from some of your contacts, then the sender receive a notice that your mail has bounced because the Sender IP address has been blacklisted.
As part of our mail server spam protection we use a Realtime Blacklist service (RBL) to identify and block mail from suspected spammers.
The Sender IP is checked against many DNS based email blacklists (Commonly called Realtime blacklist, DNSBL or RBL). In case the Sender IP is blacklisted our mail server will reject that email and the message will not be delivered to you.
Example: firstname.lastname@example.org (after RCPT TO): 550 Sender IP (XX.XX.XX.XX) is blacklisted at new.spam.dnsbl.sorbs.net
The case is often confused with the problem upon receipt while it is entirely issue with the sender, related to their IP poor reputation at the given realtime blacklist database.
Unfortunately it is not secure to whitelist senders IPs so if you would like to accept email from such senders you can disable the blacklist protection per mailbox from the Email Accounts section in your hosting control panel.
Case 2: Not Sending
In other occasions, when using our SMTP server you might not be able to send emails and the error looks like:
server response was: 550 Sender IP (XX.XX.XX.XX) is blacklisted at new.spam.dnsbl.sorbs.net
The issue is this case appears because you are trying to use our SMTP without authentication and your ISP IP is blacklisted.
The solution in this case would be to enable the "authentication when sending" function of your mail client.
- Outlook: enable the check-box "My server requires authentication"
- Eudora: enable the check-box "Authentication allowed"
- Mozilla: enable the option "Use name and password"
- Mail for Mac OS X: enable the check-box "Use authentication when sending"
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