We have a windows dedicated server with 1&1 using MailEnable as Mail Server, and since yesterday we are trying to send emails a customer, and every emails we send bounce back with that error: "Remote SMTP Server Returned: 451 Internal error"
Does anyone can tell me if it's really the remote server who is having a problem or the problem is on our side?
If the problem is on our side, how can i troubleshoot that problem?
I'm not really familiar with email servers...
We are able to send emails to other customers, hotmail and gmail addresses also... so my guess is that our customers is experiencing some difficulties but i just wanted to make sure that the problem CAN'T be on our side.