Look in the Global Settings/IMAP tab for the settings (& MTA tab for Authentication). BTW, the correct port for Submissions is 587 not 25, to use that you would require a change to the master.cf file. If you want to make that chang, do the following:
make sure that the whitespace remain at the beginning of lines 2 & 3. You might also want to vote on this bug. Changes you make to the master.cf file will not survive an upgrade, you'll need to make the change again.
In the /opt/zimbra/postfix/conf/master.conf file uncomment the following three lines:
#submission inet n - n - - smtpd
# -o smtpd_etrn_restrictions=reject
# -o smtpd_client_restrictions=permit_sasl_authenticated,reject