Ah ok, add your vote to
Bug 23191 - rtl (right-to-left) not supported
You may also be able to change:
/opt/zimbra/mailboxd/webapps/zimbra/skins/_base/base2/skin.css
BODY { margin:0px; }
to:
BODY { margin:0px;direction:rtl; }
/opt/zimbra/mailboxd/webapps/zimbra/skins/_base/base/skin.css
body { @ChromeBg@ }
to:
body { @ChromeBg@;direction:rtl; }
EDIT:
Unfortunately while this changes pretty much the entire interface it doesn't affect the compose area.