I exported my emails from my email service provider and uploaded it to mailfloss. Will it sync back to the ESP?



No. mailfloss has no way of knowing that the emails came from your ESP. There's most certainly a better way to do this with mailfloss. mailfloss was designed so you would not have to do any file exporting and importing from/to your ESP.

Under your integration inside of mailfloss, click "Show more options" and then click "advanced". You should see either lists or tags there. It is better to either group your contacts into these lists or tag the contacts and then do an import form mailfloss. 

This will ensure any actions you want to do from mailfloss will be applied to your contacts from the connected ESP.

If you have any questions on best how to do this, please contact us. But if your first thought is to export a file and upload it, that's most certainly not the "mailfloss way".