Let's see if we can break this down.
This is what happened to me. I made sure 25, 443, 465, and 587 were open in the firewall, but email was still being blocked with similar messages. I contacted IONOS (my hosting provider), and they told me that port 25 blocks any mail attempt and it needs to be opened before 587 would work. I think is stupid (or a croc of $h..) but here we are, they opened port 25 and everything is working again.
550-Requested action not taken: mailbox unavailable: This indicates that the recipient's email address couldn't be found or is inaccessible on their mail server.
550-Sender address is not allowed: This means the recipient's mail server has blocked your server's sender address or domain.
550 1MspyA-1tdm9M1E6a-012smp: This is a unique identifier for the error on the recipient's server, not very helpful for troubleshooting.
Here's a breakdown of potential causes and how to troubleshoot them:
Recipient Email Address Issues:
Typo in the email address: Double-check for any typos in the recipient's email address. Even a small mistake can cause this error. Always start with the simplest solution, and also make sure the mailbox exists and isn't full. (I am sure you have already tried these, but it doesn't hurt to double-triple check)
Sender Reputation and Authentication:
Sender address is blacklisted: Your server's IP address or sending domain may be on a blacklist due to past spam complaints or issues. You can use tools like MXToolbox to check if your server's IP is blacklisted.
Lack of proper authentication: The recipient's server may require authentication (like SPF, DKIM, or DMARC) to verify that your server is authorized to send emails from the sender's domain. Make sure these authentication methods are correctly set up for your domain.
Reverse DNS issues: The recipient server might be checking for a valid reverse DNS record for your server's IP address. Ensure that your server has a proper PTR record that matches your domain name.
Server Configuration Problems:
Firewall issues: A firewall on your server or the recipient's server might be blocking the connection or specific ports needed for email communication.
Incorrect MX records: MX records tell email servers where to deliver mail for your domain. If your domain's MX records are incorrect, emails might be sent to the wrong server.
Relaying issues: The recipient's server may be configured to prevent relaying, which is when a server sends an email to a recipient that's not hosted on that server. This is a security measure to prevent spam.
Other types of troubleshooting
Check your server logs: Look for any error messages or clues about why the email was rejected in your mail server logs (e.g., /var/log/mail.log).
Contact the recipient's email provider: If you suspect the issue is on their end, you may need to reach out to their email provider for assistance.
Once you get it working, don't forget to:
Use an email testing tool: Tools like Mail-tester.com can help you diagnose email deliverability problems and identify potential issues with your server configuration or sender reputation.
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