I believe if you have set up your phone to have the SMS messages in a different application (i.e. not mixed with emails) you will not delete your emails.
I believe if you have set up your phone to have the SMS messages in a different application (i.e. not mixed with emails) you will not delete your emails.
Messages - Menu - Options - General Options - SMS & Email Inboxes (seperate)
Now when you delete all your SMS, the mail stays put and vice versa.