Proof of posting?

Hi, I received a claim from my tenants solicitor via the county court. I received this last Monday (9/6/25) I have 14 days to reply and request a further 14 if I wish. The problem is, the court paper has a stamp on of the15th of Feb !.. It came through the post, no date stamp on the envelope, must of come through a franking machine, How on earth can i prove I have only received this?
Also i would of though i would have heard something by now if they thought they had sent out when they did.