Hi, we have a VPS and a few domains with webhosting and using sendmail.
We had to point one domain's MX records to another non-sendmail server. Now when anyone on the old server(sendmail) send an email to that domain with the changed MX record, the email errors with the following message:
"There was an error sending your message: unable to add recipient [firstname.lastname@example.org]: Invalid response code received from server" However, you can email that person from yahoo, etc. Only does not work from the server that houses their domain and website. This has been the case for over a week.
To make things more interesting, this client uses sendmail to process an online form and now it doesn't work.
Please add the following record to Server > Mail > White list in CP:
127.0.0.1 / 8
If you have other 127.* networks added there then replace them with the above record only.
OR run this command to your server ip to check that smtp is running
telnet x.x.x.x 25
It may be cause of a wrong A record in the configuration file.