According to qmail's description of the mbox format (http://www.qmail.org/qmail-manual-html/man5/mbox.html),  the 'from' lines shouldn't contain timezone info, but may contain additional information after the date.  So I think a better change is just to add [^\s]*\s* to the end of the pattern.

Note that the docs recommend the PortableUnixMailbox class as preferable for just this reason: there's too much variation in from lines to make the strict parsing useful.

Change committed to trunk in rev. 53519, and to release25-maint in rev. 53521.  Thanks for your report!