Get Unique Id From MailItem (Microsoft.Office.Interop.Outlook)?

There is no such property. You can create your own property using MailItem.PropertyAccessor or MailItem.UserProperties, but it will stop being unique if a message is copied to another folder as you will now have 2 items with the same id.

Unique Id for MailItem can be obtained by combining few email properties that could unlikely be the same for any different emails e.g.

$"{item.SenderEmailAddress}-{item.SentOn.Ticks/TimeSpan.TicksPerSecond}-{item.Size}";