Why does OWA remove text and colon from subject line of emails?

If I send an email with the subject of "dev: Testing" (without the quotes) The email is displayed with a subject line of just Testing (no dev:) in OWA (Outlook Web Access). Why? How can this be prevented? Seems like maybe a bug to me. 

Note that the "dev:" prefix is present in Outlook for Android. I do not use the actual outlook application, so I don't know what behavior is exhibited there. 

Thanks,

-Mikey