You should distinguish RCE and the ability to mount working, reliable exploits. RCE is possible here, and its best to consider that a fatal security failing in and of itself rather than hoping a particular set of countermeasures will be completely effective.
NX pages would make some classes of RCE exploit difficult or hard (such as including shellcode directly in the POP3 response, and returning directly to it) but doesn't help for many other classes (return-to-libc, or finding or installing 'gadgets' elsewhere in executable pages).
ASLR is a working countermeasure for other classes of exploit, as long as addresses are unpredictable enough to make a search is hard.
NX pages would make some classes of RCE exploit difficult or hard (such as including shellcode directly in the POP3 response, and returning directly to it) but doesn't help for many other classes (return-to-libc, or finding or installing 'gadgets' elsewhere in executable pages).
ASLR is a working countermeasure for other classes of exploit, as long as addresses are unpredictable enough to make a search is hard.