Am prevented from replying to purchaser, the message just says invalid message (received I think?)

[sacsamhb]

I am trying to reply to a message received from a purchaser trying to arrange pick-up of goods.

I able able to compose my reply but cannot send, I have an Ebay message window saying that the received email is older than 90 days (it is one day old) or "invalid". 

I can't see why the received message is invalid and I need to contact the purchaser so I can deliver on sold goods.