When trying to email certain domains, you receive the error below even though you have authentication turned on in both your SmarterMail server and your mail client:
Failed Recipient: firstname.lastname@example.org
Reason: Remote host said: 550 Please turn on SMTP Authentication in your mail client.
mail.domain.com [xxx.xxx.xxx.xxx]:xxxxx is not permitted to relay through this server without authentication.
This happens because the DNS servers that you are using in your SmarterMail configuration are unable to find the MX records for the server that you are sending to. Since this is the case, SmarterMail tries to send to the A record of the domain you are sending to. If the A record points to a different email server besides the one you are trying to send to, you will receive the error above. There are 2 resolutions to the problem:
1.) Clear the cache of the DNS servers that you are using in SmarterMail. If this does not work, you must troubleshoot the DNS servers to find out what may be causing them to not be able to find the MX records.
2.) Change your DNS servers in SmarterMail to Google's DNS servers which are 22.214.171.124 and 126.96.36.199.
Once you complete numbers 1 or 2 above, restart your SmarterMail service and this will resolve the issue.