Tayo writes, "Our Outlook client suddenly stopped relaying mails with the error "Sending' reported error (0x800CCC78) : 'Cannot send the message. Verify the e-mail address in your account properties. The server responded: 550 5.7.1 This system is configured to reject mail from (IP ADDRESS) (Host blacklisted in uce3.dnsbl)' "
The IP address (IP ADDRESS) was traced to our ISP source. I think somebody blacklisted their IP Address. We called and they had been working on it since three days now with no solution yet.
Can you be of help? What can be done from ourside to solve this problem?"
Tayo, thanks for writing. It sounds to me like your ISP is blocking your own legitimate emails due to incorrect configuration on their part. It sounds like they use the UCEPROTECT blacklists, but haven't configured it in a way that would exempt their own customers from being able to send their own mail.
You're on their network. You're connecting to their SMTP server. This isn't a connection to an outside server. This is, very simply, the proper way for you to send mail. Maybe another ISP might block your mail (if they so choose), but your own ISP's mail server is supposed to be configured not to reject mail originated from legitimate connections on their own network.
Unfortunately, this is something you have to have your ISP fix for you. It's probably not something you can resolve on your own.