I administer a web server running Debian 5.0.1 on an Amazon EC2 instance and it uses exim4 for sending email. One of the sites on this server sends periodic emails to an email address: firstname.lastname@example.org. A couple of days ago these emails stopped getting through. I changed the email address it was sending to my own email address, email@example.com and I received it successfully. I then tried to send an email from the command line on the server to each of these addresses. Again, I received the email while the first recipient didn't. I tried other email addresses @domain1.com and they are all failing. After checking the exim4 log I saw the following error message:
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>: host mx01.1and1.co.uk [xxx.xxx.xxx.xxx]: 451 Requested action aborted
I've googled, but I can't seem to find any info on that specific error. Can anyone suggest a way forward for me?