julienlg have you checked your spam folder?
I think that this isn't an internal server error, since last logs/errors are from 25th March at your forum. These do not show any E-mail related errors.
Specifically, what doesn't arrive? When new post is created in discussion you're following, or when you're signing up, resetting password etc...? It could also just simply be rejected by your mail server/provider, in that case I can't help you