Apple Mail (nor Thunderbird mail) cannot send

Question

Level 1

15 points

Apple Mail (nor Thunderbird mail) cannot send

cannot send mail but can receive mail

This is a message I get on Apple Mail:

"Could not connect to this SMTP server. Check your network connection and that you entered the correct information in the Account preferences. Also, verify that the server supports SSL. If it does not, deselect the "Use TLS/SSL" checkbox in the Server Setting tab of Account preferences"

Everything checked out OK that I could see on the Accounts/Server settings page: the host name, user name, password the port numbers. Except under Outgoing Mail Server (SMTP) Account says "Offline." All the other boxes are filled out correctly (user name, password, host name, port number).

The message I get when trying to use Thunderbird mail:

"Send Message Error

Sending of the message failed.

An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: [C18] RBL restriction: Blacklisted by Internal Reputation Service - 66.119.222.230."

What does any of this mean, and how do I correct it?

Thank you.

iMac (M1, 2021)

Posted on Nov 21, 2025 7:15 PM

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Answer 1

AlWeir

Level 5

4,862 points

Nov 22, 2025 9:51 AM in response to susandf

Your Outgoing Mail Being Rejected (The 'C18' Error)

The term [C18] RBL restriction in an error message often refers to your own outgoing mail being rejected by the recipient's mail server.

Scenario: You write an email in Thunderbird and click "Send." Your email is sent via your Outgoing Mail (SMTP) server.
Error: The recipient's server checks your SMTP server's IP address against its RBLs. If that IP is blacklisted (which can happen if a server or network has a history of spam), the recipient's server rejects your email, and you get an error message like:

"An error occurred while sending mail: The mail server sent an incorrect greeting: 5.7.1 [C18 RBL restriction: Blacklisted by...]"

Solution: If you encounter this, the issue is typically with your Internet Service Provider (ISP) or Email Host's server IP, not your Thunderbird settings. You would need to contact your host or ISP to resolve the blacklisting.

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Answer 2

susandf Author

Level 1

15 points

Nov 22, 2025 1:42 PM in response to AlWeir

Thank you! I will contact my internet provider today, and my email host on Monday when they will be open. Interesting that I got detailed information from Thunderbird and really nothing helpful via Apple Mail.

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