I wouldn't want to use 5 different blends since we need to adjust our IMAP backend too. Zend Framework or ez Components offer pretty good code as well.
E.g.: http://ezcomponents.org/docs/tutorials/Mail http://framework.zend.com/manual/en/zend.mail.html
They also support reading mbox files (in case your RoundCube and mailstore are "closer") and come already with a driver based architecture, code is follows established coding guidelines and standards and is also documented etc.. Let alone the license (BSD) allows us to do anything and is generally more liberal than (any) GPL. Etc. pp. - things to consider.
I'd welcome using ez components. Less issues to un-glue the mail component from the rest and they also have a pear channel etc. which rids us of all dependency issues.
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