[RCU] Strange behavior while sending mails

Hi everybody,

I have configured roundcube to access my mail server through the submission port. I have enabled also the smtp debugging and I get the proper log if I send a mail using roundcube. In the log, I get these last messages telling everything is ok: 

[10-Jul-2014 22:32:26 +0200]: <k2d6nftq> Recv: 250 2.0.0 Ok: queued as 3C1A7E02DB [10-Jul-2014 22:32:26 +0200]: <k2d6nftq> Send: QUIT [10-Jul-2014 22:32:26 +0200]: <k2d6nftq> Recv: 221 2.0.0 Bye

However, in the web interface, since I press the button "send" that appears a toast message saying "Sending mail..." and never stops until it is replaced by another which reads "Timed out". So it is like there is some problem by properly processing the output from my smtp server in the roundcube side.

Does anybody know how to solve this issue?

Thank you very much in advance, Felix