I've always worked directly with Dovecot and Sieve - I've never seen roudcube as I've never needed anything else.
Dovecot may be a bit more "manual," but it's quite easy to write up scripts and test them against sample emails to see what would have been done.
As such you can set up replies, receipts, forwarders, trashes, etc for both system-wide and user-centric scripts. Not only that, but once you get familiar with that, you can them jump back and forth between postfix access-lists (such as IP filers or Domain filters) and customizing Dovecot's error/status reporting when NDRs or rejects *very* granularly.
If I get an email from Afghanistan, I can reply with "Durka Durka!" If I get an email from foo@bar.com I can reply with "It's 'FUBAR' due." Or general Dovecot error messages. I assume all that roundcube does is script up the same.
In a pinch, you can even telnet into Sieve to fix something when you are away. I use a Perl script to manage it and it's a snap.
If you want to go in that direction, I'm happy to help and can even post examples. Some references are:
http://fastmail.wikia.com/wiki/SieveExamples
http://fastmail.wikia.com/wiki/SieveRecipes
http://www.michael-hammer.at/server_config/mail_filter/
http://wiki2.dovecot.org/Pigeonhole/Sieve/Examples
That should get you started if you are interested.
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