XP, ffox 3.0.3, simple mail 2.71

I am getting error 553-5.1.2 intermittently when sending To multiple recipients from simple mail 2.71. The syntax looks fine. Happens in both reply and new mail. I am able to send these message directly from gmail interface. I have searched for solutions but have had no luck.

Also is there a location where I can view the error message aside from trying to memorize the 3 second pop up on the bottom of the screen. I've looked in the error console but I cant see it .

This is a great extension but the bugs make it hard to work with sometimes.

Thanks