I just started using Simplemail and I noticed that when I reply to some email addresses I get a popup box telling me "invalid response, undefined email address" and error 550.
By trial and error I worked out that Simplemail doesn't like email addresses with a comma in the name.
John Smith - is OK
Smith, John - gives an error
A simple fix for the next version I hope. Apart from that, I'm liking Simplemail. It's great for accessing my email at work. It gets around my ISPs webmail service which falls over regularly and it doesn't interfere with my mail client at home.