Solved! If you have a folder named: something like "sent" (Sent, sent-items, zz_Sent) then it doesn't function. Maybe the application tries to save the mail in this folder but something goes wrong. I have read somewhere that the mail application use "burl" and this function isn't implemented in a lot of imap servers. So I deleted my mail account on the device (you HAVE to delete the account)...renamed "Sent" to "Zent"...re-created the account on the device...now I can send mails. That's really strange!