To my unknowledged mind, FvwmTaskBar only polls a local file for new lines.
Simple. FvwmTaskBar didn’t work for my procmail-driven setup all that well. If I remember correctly, the manpage for it should give you more insight as to why.
But, if you have one simple traditional mbox style spool file living in /var/, and evolution isn’t fetching mail from remote machine (and if it is, have a look for a program called ‘fetchmail’), then you should have no problems with FvwmTaskBar’s mail notification (apart from, again, iirc, you have to manually middle-click the mail icon to let it know you have read your new mail in evolution)
So, based on the assumption you get the mail eventually to local machine (ie. in the machine where FvwmTaskBar is running), and mail receiving services are putting things into their default locations, my advice is to try out not defining the *FvwmTaskBar: MailBox at all.
Also, the polling sequence could not be so dense that you wouldn’t get the notification right away. Wait a minute or two.
I have now installed fetchmail and used the fetchmailconf to set up the program to get my Yahoo mail. And when I send a mail to myself (using evolution) and use fetchmail to get it, the mail seems to be put in /var/mail/me.
I have then made Evolution get the mail from this local file (/var/mail/me) and it works fine. In Evolution it even says that its an mBox format.
I have then made the following in my .fvwm2rc file: