This is a VERY good question HarryAndDracoAreHot. We don't know what was in that letter.... ??
Interesting too is the fact that we know that Dumbledore wrote to Petunia before the night the Potter's died. Now what was that about????

AS we have talked about on other threads: Why did Dumbledore correspond with Petunia? We know she isn't magical or even squib (JKR said).
How/why does Petunia know about Azkaban partiicularly? (Even if Peti. was just over hearing, Azkaban seems like a strange topic for the James and Lily to talk about OR for Peti. to remember.... ??)
BUT the real clincher is, WHAT WAS GOIN' ON BETWEEN D. AND P.???? Really weird!!