MobileMe email - Illegal host/domain name found

have a bit of an odd problem with MobileMe email in that sometimes emails to one particular email address bounce back undelivered with 'Illegal host/domain name found'. The email address is a perfectly valid one, and sometimes the emails sent from my iPhone are delivered fine. I never have a problem sending to this email from my Mac, although this uses a different SMTP account than the MobileMe one which makes me think the problem is with MobileMe.

Here's the full error message for an email I sent to two email addresses, mail@xxxxxx.com being the problem one and mail@yyyyyy.com being a different one (though both recipient email addresses are hosted on the same server). The message to xxxxxx bounced but the yyyyyy one was delivered fine.


Any ideas greatly appreciated!