Path to a directory to store data used in verifying AsyncMDNs.
If Post is invoked after setting AsyncMDNInfoDir and an asynchronous MDN is requested, the class stores the data required to verify AsyncMDNs in a file in the specified directory. The name of the file is the MessageId of the outgoing message.
AsyncMDNInfoDir is also used while verifying asynchronous MDNs using VerifyReceipt. The properties required to process AsyncMDNs, namely OriginalContentMIC and MDNOptions, are automatically read from the file saved at the time of sending the original message.