What was the letter that Pluto was holding in the after credits scene?

Pluto is shown holding the second (of three) letters from Mickey. It is shown both in the credits of KH 1, and in the intro to KH: CoM. But the contents of the letter were never revealed, nor was the source. Nomura has stated in interviews though that when Pluto left Sora in Traverse Town he traveled to a few different worlds and did meet with Mickey, where he got the letter.

In fact, while it is shown that Sora, Donald and Goofy chase after Pluto, Pluto is not shown to follow them to Castle Oblivion, so it's entirely possible they never even caught Pluto to get the letter from him.

But for now, it remains a mystery in the KH Universe. A bit of my own speculation is that given the way things transition sharply in Chain of Memories, it is entirely possible there was a different idea how a sequel might continue at that point, which was later changed. Which would explain a number of things at the end. But that is all conjecture. What is known is that we don't know what the letter says.