Porn sites send emails out in bulk to lists of all the emails they can get hold of, that is the most likely was that they have your email address.
Do they seem to be from the attractive neighbour? Block her from your emails, then you will have no more from her.
If they are not from her, how do you know that she has anything to do with it? Unless you share your email with your husband, you will not get one another's messages.
If you don't share your passwords with him, why was it that "Our devices share the same wifi and he kept mentioning one of my passwords in conversation which I have changed as a result. "
If you are still not happy about his relationship with this woman, have you asked him (without accusing him of anything until you have more evidence. but with a laugh) something like, "Sharon Blank seems to fancy me - or someone with the same name does, because they keep sending me pornographic emails. Well, I don't fancy her!" If he has a guilty conscience you will soon know.
This is so like a previous thread from someone whose husband with Altzheimers was obsessed with a neighbour that I really can't take it seriously.