If the iPhone is using an IMAP account on your server and that requires authentication to send mail then there should be no problem of it being checked as spam. Perhaps you could try this, enable port 587 for submissions and modify the iPhone to use port 587 for submitting email - that will require the user to authenticate for sending mail.
You set port 587 by doing the following:
uncomment the three lines (leaving the white space on lines 2 & 3) and save the file and restart Zimbra.
in /opt/zimbra/postfix/conf/master.cf.in at the top of that file you'll see the following lines:
#submission inet n - n - - smtpd
# -o smtpd_etrn_restrictions=reject
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject