There are any number of reasons emails go into your spam folder. Email programs have gotten very "smart" and they try to help you manage the abundance of email we now all receive. Certain words in an email to can trigger it to be sent to the spam folder even if it's something you signed up for. Certain senders can also be identified as a spam sender if someone has complained about them or if their pattern of sending triggers an email provider to think that they're spam based on their algorithm.