You are making the message recursive.
message.Body = something that embeds message
When you call ToString() on it, the message gets written first to a MemoryStream (and from there, gets converted into a string) and the MemoryStream buffer must continue growing without bounds because there is no end to a recursive message.
You likely meant to embed a different message, but your code has a body part of the message pointing back to the top-level message again resulting in an infinite loop when it gets written out.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…