I ispconfig3 server works really well having followed the lenny perfect guide. I came across a problem now with the email that i'm having trouble understanding or fixing. The emails work great with a few domains - however, on a domain that has been settled in dns for a week or so now, and the sites work fine, there is a problem with the email though. I created the mail box for firstname.lastname@example.org. It works fine when sending emails to it from google mail, yahoo and a couple of others. However, using one particular mail server i get sent back the message The user does exist at that domain and can successfully receive emails from other senders. Is this likely to be a problem on my side or the mail sending client?