Roundcube - it's just web-based mail program. Like outlook, thunderbird, iOS mail, gmail and etc.
You must check and tune your SMTP server for spam filters and etc.
For example install postgrey for postfix, do check DKIM,DMARK, SPF records for senders domain and mail hosts. Install spamassasin. Use black list from open source spam list projects.
About your phone. Ask your IT or provider about IMAP, SMTP settings of your mail server and config it on your phone.