There are a number of reasons why this may be happening. First and foremost, your data collection practices are going to be the driving force behind your inbox delivery.  If you are collecting email addresses on a web form then you should make sure of the following:

1. On the web form you inform people that by submitting their email address they agree to receive X number of mailings from you per week or month
2.  On the web form if you have a check box that once checked will opt subscribers into your mailing list then this check box cannot be pre-checked
3. Upon sign up we strongly recommend that you send a confirmation email to the email address that was entered.  This is a feature you can turn on from Manage Forms>Custom Forms>Pencil icon on the right and then Opt-In Confirmation set to ON
   

Sometimes even if you follow the above data collection steps it is possible some of the ISPs may consider the message to be spam and thus send it to the junk/bulk folder. Some of these reasons could be:

1. ALL CAPS in the Subject line
2. Too many special characters such as $%^&* in the subject line
3. The words FREE, MONEY, SEX or any number of 'trigger words' could send your mail to the junk folder or Spam box.
4. Your content could be considered inappropriate
5. Your mailing consists of just one large image and no plain text to accompany it.
6. If you have links in your email, it is also possible one of these has been blocked by the ISP and is therefore being tagged for the junk/bulk folder.