Problems Sending/Receiving Mail with a Particular Domain

I am using Postfix and haven't made any changes my server configuration in a while. Recently I am having problems sending/receiving mail to hp.com. It all worked fine some weeks ago. The error I am seeing in my logs is:

Jan 25 10:57:09 wilkesley postfix/smtp[28009]: 637DECA86: to=<notmyname@hp.com>, relay=smtp.hp.com[15.201.24.91]:25, delay=303040, delays=303036/0.02/0.65/3.2, dsn=4.1.8, status=deferred (host smtp.hp.com[15.201.24.91] said: 450 4.1.8 <notme@manor-farm.org>: Sender address rejected: Domain not found (in reply to RCPT TO command))</notme@manor-farm.org></notmyname@hp.com>

I think that this means that hp.com can't find the domain manor-farm.org, so their server is rejecting the message. Other people can send me email and as far as I can tell my DNS (on Linode) is correctly configured.

Is this a problem at hp's end, or with some intermediate DNS cache.

Ian.