I am trying to discover if there is a way to determine the internet message ID after sending an email using the EWS Managed API. I understand you can go in there and get the res
No you can't, basically because EWS sends message Asynchronously the Id isn't available see https://social.msdn.microsoft.com/Forums/azure/en-US/dd034b8c-ffa1-4ae0-9025-45fcf520c9e5/updateitem-does-not-return-itemid?forum=exchangesvrdevelopment
As a work around you might want to consider setting the Internet messageId on the Message before you send it. As long as it valid and unique it should work okay eg
ExtendedPropertyDefinition PidTagInternetMessageId = new ExtendedPropertyDefinition(4149, MapiPropertyType.String);
EmailMessage ema = new EmailMessage(service);
ema.Subject ="test from ews";
ema.Body = new MessageBody("test<br>Rgds<>");
ema.ToRecipients.Add("gscales@domain.com");
ema.SetExtendedProperty(PidTagInternetMessageId,("<" +Guid.NewGuid().ToString() + "@domain.com>"));
ema.SendAndSaveCopy();
Also if you save the message first as a draft before sending it the server will assign the MessageId property which which should then be able to read back using Load.
Cheers Glen