Sorry if my English is not good enough. but I'll try to explain the problem.
when I sign in the program shows me for a moment the mailbox and then drop out back to the sign in page.
Did somebody the same problem?
setup was done according the instructions to postfix program...
Is there anything in your error log?
Which log file should I look in? (in Linux)
In your [RC root]/logs there should be file titled errors