Since Ticket #15138 has been fixed, it became possibe to make some changes on http://wordpress.org/extend/plugins/mailpress/download/ .
5.0.1 remains the current version
Development Version hosts the current mailpress development version (ah ah!).
In fact this is the first beta release of MailPress 5.1 for tests only.
This version has been tested with WP 3.0.x and WP 3.1 nightly builds.
Do not hesitate to report any bug in the comments area giving your wp version
Thank you !