DL has enough of an international presence at BOS that they do have a recheck desk in the customs area, and they should be able to check you in and take your bag. The only question is whether it will be staffed when you arrive; if it's not, then you will have to schelp your bags to A. If you have to do that, I'd recommend walking to A if you can. Basically, after exiting the CBP area, go to the top floor of terminal E and then walk 10 minutes through the parking garages using moving walkways. It's well signed and much more reliable than taking the inter-terminal bus.