I have been using Notify.app since earlier this afternoon. It cannot send e-mails with Notify.app with either of my Yahoo accounts, but it receives e-mails okay from Yahoo. Notify.app receives and sends well with Gmail. When the Notify.app is configured to open up an e-mail with my default Mail.app client, it marks the e-mail as read -- which overrides my user preference of don't mark as read (TruePreview add-on). So if I do not send an e-mail using Yahoo with Notify.app, and if I do not open up my e-mails from Notify.app with my Mail.app, it is a workable solution.
I bought Sparrow today, but it automatically marks e-mails as read and doesn't show the read / unread count in the inbox.
The mailtab option in iTunes recommended by cuestakid is a wonderful option for people with only gmail accounts.