POBoOX for defendant problem?

[isitnow]

What problems are there from using a POBOX address for the defendant's address when making a claim against them?

The POBOX does not lead to a real place of buisiness.
They have acknowledged service so I presume service cannot now be declared invalid?

Are there any pitfalls in claiming against a POBOX address and does it need to be changed to something else now that the case is underway?
What can go wrong if it is not changed to a proper address? I cannot start the claim again!