Watch out with that.
It doesn't feel right to host windows-live or hotmail together with gmail under mail. Because windows-live or hotmail is closer to Microsoft and might allow more things to be done than the other, especially with regards to things like the Marketplace, SkyDrive and what else requires a Live ID.
The same goes for windows-live vs exchange, the mail client on the Windows Phone functions quite different between both. For instance, I can't flag my mails on windows-live while my father can do that on his exchange account. Also, exchange isn't using IMAP like windows-live does; which could bring other differences along as well.
I also think that google shouldn't be synonymous to gmail, because you could ask questions about something like Google Docs on the Windows Phone or perhaps Google itself. This could be resolved by retagging google questions to one of gmail/gdocs/google-search.
As for Yahoo, I would go for more specific tags (yahoo-mail/yahoo-calender/...) such that they are consistent with how Google tags are used. I'm not quite sure though, but we need to consider:
Is the yahoo tag useful for searching? Who would search all yahoo questions?
Is littering the mail/calendar/... tags with Yahoo questions OK?
Do we want to make it harder to search for yahoomail or easier using yahoo-mail?
These three questions could also be useful for deciding how to tag the other questions; to summary these questions think about categorization, search-ability and what the tags contribute to the question.
So, when we plan to retag any of the above, we need to be careful with how we approach it.