it doesn't matter what your connection priorty is, you can set it in your email setting under the 'account setting' -> acount information -> 'access point '
OR if you can turn off the sync under the  "When To Synchronise"
OR you can specify what time and day to sync under the "Timed Synchronise".

if i'm not wrong, once you set "Synchronise email" to 'OFF' it won't pop up again... i think lah... (but then what's the point you setup the mail if you don't let it sync??)

note: if you have more than 1 mail account, then you must set every account...