Since it's impossible to send hundreds of pictures attached to an email (there's a 25MB limit), turn off automatic download of attachments and just manually download each attachment once. (All you have to do is tap it.)
If that still gives you hundreds of downloads, delete your email account from the email app, then add it again, as an existing account. Let the app set things up, don't set them manually.
If it still keeps giving you hundreds of downloads, install a different email app, put your account into that one and delete it from the one giving you problems.
(If a second email app does the same thing, you have major problems with the phone's firmware
or with your email provider. Check with your email provider first. If that's not it, first
back everything up. Then, if the phone's still under warranty, bring it back, or arrange to send it back (depending on how you got it) and have it fixed. If it's not under warranty, reflash the firmware (see
[Samsung] How to flash Stock ROM via ODIN), then restore the backup.