MessageLabs anyone...?

[#1]

We've been referral partners with MessageLabs for a short while and it seemed to make sense that we should use the service ourselves to give additional confidence to our customers.
Spoke to various people in support about this and have been getting mixed reactions so we're really no longer sure whether it will work for us or not with PlusNet and I wondered if anyone else was using MessageLabs and could offer any suggestions.
Can I just quickly add we don't have a problem with PN's AV or Spam services, it's purely a business reason for shifting over.  I appreciate that Postini offers similar functionality.
We don't have a need for an internal mail server so just use standard POP3 collection and have two domains hosted and registered with PlusNet.
How MessageLabs 'should' work is that our MX records are changed to point to the MessageLabs scanning tower instead of the default PN mail server at which point they scan our email and then forward the clean messages back to PN's mail server.
PN have now given us two different IP addresses for the mail server but from yesterday afternoon until earlier this morning it was working fine.  Emails to our domain were hitting the right MX records, being scanned by MessageLabs and then returned to PlusNet where we were able to pick them up and, indeed, we had a number of legitimate emails this morning and no spam.  However, sometime between 9am and 9.30am this stopped working and we don't know why.  So all day we've had no emails cause people trying to contact us have been getting a bounceback saying too many hops (apparently emails are now looping but no-one's changed anything between last night and this morning) or they're getting no error message at all but we're not getting the email.
<Sorry, this is getting a bit longwinded>
So my question is...  are any other PN business customers using MessageLabs and, if so did they have any problems in setting it up.  Any pointers gratefully received.  Bear in mind we're not operating our own SMTP server but using POP collection.  Some PN support staff say this won't work, others say it will so we're getting mixed signals.

[#2]

One of my colleagues raised your issues with me shortly before leaving the office this evening.
My understanding from memory is the method you are attempting is not possible with our POP3 setup.
In POP3 mode, our servers cannot be removed from the MX records and as such, not all messages will go through Message labs as a result.
So whilst technically possible, the reasons for doing so cannot be fulfilled properly.
The only true way to do this, is if you are running your own destination SMTP server

[#3]

I think what might work is if the MX records for domains were pointed to MessageLabs and then MessageLabs configured to forward the mail to mailbox@username.plus.com. You'd probably want to configure your account to completely opt out of Postini.