avasout02 blocking email to valid address

[#1]

Hi,
I have my own mail server for business use (totally unconnected to PN, but may move the business broadband over shortly).
I believed that I had set up a specific user with an alias for a particular email address - so I tried sending mail to it from my personal PN webmail account and received a User Unknown rejection message.
Upon checking the server, I had not actually set up the alias that I thought I had.  So I set it up and tried again.  Same result, received a user unknown rejection error instantly.
I had a colleague send an email from their hotmail account to the same email address, and it is received successfully on the server.
Is avasout02 caching the previous unsuccessful result and simply not attempting to deliver the mail now?  If so, does anyone know when this server at PN will remove this error and recheck to see if the address is valid again?
Thanks,
Chris.

[#2]

Well over a week, and avasout02 is still blocking email to a valid address.  Tested from hotmail and the mail arrives fine.  Test from plusnet webmail and avasout02 has a cached response of 550 :User Unknown.    This is not the case.  The user is valid, and email is being blocked/rejected incorrectly.
How long does avasout02 cache its results for?  Is there a way to force this server to try looking up again?

[#3]

It'd be the first I heard of a mail server caching a 550 error.
Assuming you don't want to post the full mail address, what's the domain name? Are you sure your DNS is, and has been, watertight and not sending Plusnet somewhere different to Hotmail?
Mathew