I do know for certain that on ICCF engines are allowed, but the question is whether analyzing the position with a human is allowed? I know that on chess.com if you let someone play on your profile you will get banned. The reason is that if it's allowed I am planning to post a question regarding my current position on a game on the ICCF.
Yes, it is legal.
ICCF: Says nothing about it, or assistance of any kind, and you can even use computers legally. Here are their rules.
USCF: "3. You may consult chess books and periodicals but not other players." Here are their rules.
With regards to the ICCF rules, my guess is that they just decided it was too hard to police computer used, so they just allow it. Books have always been permitted. Human assistance also.
Whent I played in ICCF the rules were any sort of reference help other than another person was allowed. That was mainly books back then. I had wondered if Berliner had used his IBM access to help win the world championship in the 60s.
And I understand that now they specifically allow computers as well as books.
There is no way for them to know if someone is using another person. So the only thing stopping it is your ethics and conscience.
Why bother playing postal chess if you are average and then getting someone to help you. And if you are a top player good luck finding someone who could help you better than you and your computer can do.