I have uploaded the current (and, as of yet, unreleased 5.3.28 version) of vpopmail to the CVS server on SourceForge. You probably won't be able to browse it on the web right away, as the CVS browser only syncs with the main distribution point every 24 hours.
All developers on the vpopmail project can commit changes to CVS. Anyone can pull the source from CVS. If you have occasional changes but aren't a regular contributor, you can post patches to the Patch Tracker and one of the developers will post them to CVS.
For more information on CVS access, please visit: <http://sourceforge.net/cvs/?group_id=85937> It includes excellent information on learning to use CVS, including a link to an online book about Open Source Development with CVS.
And note that I'm far from being a CVS expert! If any of the other vpopmail developers have CVS experience, I'd be more than happy for them to post some helpful hints.
Also, if you commit changes that are related to a tracker item on SF, please mention it in the notes. Something like "Fix problem with domain limits [Bug #999999]".
QmailAdmin: http://qmailadmin.sf.net/ Vpopmail: http://vpopmail.sf.net/
Info on the Sniffter hand-held Network Tester: http://sniffter.com/