When mail account is created in Plesk, Smartermail restarts. Sometimes it does not restart completely and there is the error:
[SocketException (0x274d): No connection could be made because the target machine actively refused it 127.0.0.1:17001]
The root cause is internal issue with ID PPPM-1466, which was fixed in Plesk 12.0.
Upgrade to the latest Plesk version.