Guess we better stay in english
Roundcube is not exactly a Mailserver but only a Webmail Client. Roundcube can get emails from real Mailservers (POP3/IMAP) such as Dovecot and display them in a browser.
But Roundcube is NOT a POP3 or IMAP Server! For example you can use Roundcube to read your email from a external email account (ex with your ISP) in your browser.
A real Mailserver has to be installed additionally (ex through ipkg) With the latest Firmware (0832) Synology introduced a Mailserver-Combo (Postfix for SMTP and Dovecot for IMAP/POP3). Afaik your Diskstation-Model is not compatible with this latest firmware. But nevertheless it's possible to install at least Dovecot from ipkg. If you want to do so you should check here in the forum as purzel once wrote an excellent how-to for installing Dovecot on a Diskstation