This is an issue we have experienced before. My partner's primary email address is set up as an additional mailbox on our plusnet account. She has signed up for a number of shopping and social sites to receive regular communication by email. Many of these are no longer being received, from the likes of B&Q, Lakeland, Holland and Barrett, Superdrug. She is missing out on offers by email, which she wants to receive. Seems to be a case of one person's meat is another's spam.
Nothing appears in the spam folder. Messages from Nigeria seem to have no problem getting through to her Inbox.
Can someone at Plusnet look at the email settings to see if there is anything that needs to be changed.