I guess my point of posting that (over 4 months ago ;) ) was that it happens with multiple addons, so it's not just a MailFu thing. Actually, I don't even use MailFu or the mail module in SLDataText and I still get a mail icon on my map occasionally. Not really when I don't have any mail anymore, but often when I have just emptied my mail box. I'd have to open it again for it to clear the icon.
I am getting random "new mail" notifications with both MailFu and SLMail (which would also show the mail icon on the minimap). It has been happening for about a week now. Not even when touching a mailbox - sometimes I notice it when I'm in an instance, or porting. I haven't caught when it happens.