K-9 emails not sending thru dataplan

[blubot]

hello,
i am using a Galaxy SII LTE and just installed K-9 email client. I can receive emails without problems. I can send emails using wifi connection.
However I am unable to send emails using my dataplan and get the following error message: 'java.net.SocketException: Connection reset by peer'
what could be the problem? I have the same issue with MAILDROID.
help...
thank you