The ExchangeMessageInfo doesn’t contain any such property or information that may contain the Message signature. Could you please confirm to us your requirements by elaborating a little more? We’ll discuss these with our development team for any possible implementation and update you accordingly.
The signature of a mailbox is contained within the Dictionary entry of GetUserConfiguration. The signaturehtml is used for this purpose. However, I could not find this value for Exchange account at present and would request you to please spare us a little time to investigate this issue further at our end. I’ll soon write back here with updates in this regard and appreciate your patience.