It looks like our Exchange client is automatically doing this with email from Marketo with only myname@mydomain.com as the email address. My Yahoo and Gmail clients, don't automatically do this (no active directory like Exchange), but a few of the emails do have my display name rather than my email address, but I don't believe that Marketo has a way to configure this. You should submit an idea for Marketo to provide an option for sending the display name along with the email address.