this is how i got it, though geared to Tortoise CVS, many of the settings should be similar for other CVS tools...

beginning string was
:pserver:anonymous@anoncvs.forge.jboss.com:/cvsroot/jboss
module: jboss-mail

then in tvs tortoise
i went to revision tab
i unchecked "use head branch"
i checked "choose branch or tag"
i clicked button "update list", i.e. update list of tags concerning jboss-mail
under the "branch or tag name" dropdown, it now listed just jboss-mail branches

i made sure "JBOSS_CS_1_0_M5" tag/revision was selected

then did checkout

here is the cvs string tortoise cvs used...

note: this is not the release branch, as M5 hasnt been released
In E:\jboss-versions\New Folder: "E:\Program Files\TortoiseCVS\cvs.exe" "-q" "checkout" "-r" "JBOSS_CS_1_0_M5" "-P" "jboss-mail"
CVSROOT=:pserver:anonymous@anoncvs.forge.jboss.com:/cvsroot/jboss