Xfce4 Mailwatch Plugin 1.2.0 doesn't work. You can add several accounts, but the plugin only gives a notification for the first email account.
What do you mean by "notification"? The tooltip updating to indicate messages received or a notification sent to the desktop via notify-send?
Which email providers are you using?
If you setup each one as the only email account, do you then receive notifications for that account?
I'm also with version 1.2.0, connecting to 3 different mailboxes (2 gmail and 1 hotmail), I receive both tooltip updates and notification (once set up) for all 3 accounts.