-You can install another webmail on smeserver as roundcube, sogo, new horde5 (please test before) or zarafa.
-I worry that horde is too old for safari, maybe not
-You don't need to set a vpn, simply allow in email panel the imaps service and set your router to forward ports to your sme (995,993,587,25)
E-mail settings
POP3 server access Allow private and public (secure POP3S)
IMAP server access Allow private and public (secure IMAPS)
Webmail access Allow HTTPS (secure)
-please take some minutes/hours to read the documentation, seems that it could be a great help for you and a source of inspiration for many next projects with sme :
http://wiki.contribs.org/SME_Server:Documentation