I've seen this several times as well. If it's a referral link to unsubscribe and is meant to include your email, double check it's okay. If your plus is missing insert "%2B" where it should be.

It's less common, but some websites ask you to type your email address once they realise they can't find you on record. If the "+" doesn't work in that field (server side rejected), then you can try %2B there as well.