This plugin has a major drawback, IMO, which is that while it reports new messages, it doesn't give their headers (sender and subject). This makes it somewhat useless with webmails like Gmail as the user can't distinguish if a new message is just spam.
For this reason I use Mail Notification ( http://www.nongnu.org/mailnotify/ ).
Mail Notification is fine in its features and is lightweight but its development looks like stalled and its current version in Debian (sid) is using a GTK3 interface which is somewhat buggy.
MN has two good things, IMO, which are GNOME keyring integration (something I suppose wouldn't come into the mailwatch plugin, although as an optional feature it might be interesting) and its notification, which displays all the new messages' senders and subjects (both in the notification bubble when they arrive and as a tooltip afterwards) and when clicking the 'Open' button on the notification bubble (which works fine with xfce4-notifyd) it opens directly the message. These in case of Gmail, which is what I use.
In short, I'm not a programmer, but I thought that xfce4-mailwatch-plugin could maybe easily incorporate the ability to show the new messages' headers and to open them (or the last one of many) directly when requested (I don't remember if it does that already or if, as I think, it only opens the Gmail page; anyway, this last feature is just a plus). For the reason exposed in the first paragraph, I think this is a basic feature and looking at Mail Notification it doesn't look like it would make the plugin much heavier.
*** This bug has been marked as a duplicate of bug 8462 ***