Typically, when subscribers don't see your email campaign in their inboxes, it's because of spam filters. If your recipients have looked in their spam or junk folders and still don't see your campaign, there are a few other things to investigate.
Is a particular subscriber not getting your emails? Ask them to add your From email address to their contact list or address book.
If your From email address is in their address book, have them check their spam filter settings to be sure that your content and/or address isn't blocked.
Are they using a webmail provider such as AOL, Yahoo, or Hotmail? These providers throttle delivery, which can sometimes result in emails taking about 24 hours to be delivered to a specific email account.
Has the campaign actually been sent? It's possible the campaign is still in the delivery queue for sending from our servers. If you have Mailchimp Pro, or our Premium plan, use the Delivery Insights feature to see updates on the delivery progress of your campaign.
Could there be an internal firewall? Some domains don't like seeing emails going to and from the same domain, via a third party. For instance, you may be sending an internal company newsletter from firstname.lastname@example.org to email@example.com. In the middle of that process, your email goes through a Mailchimp server. Sometimes, particularly with corporate and university filters, these emails look suspicious and are blocked. Review the safelisting information (other providers may refer to it as "whitelisting") if this may be the case.
If none of the above seem to get your emails to subscribers, they may need to add our IP addresses to their safelist. Generally, only corporate domains can do this. Consumer ISPs won't add IP addresses to their safelists on a case-by-case basis. (We work with consumer ISPs in other ways, though, to help ensure that our servers can deliver mail to them.)
Typically, subscribers should ask their server administrator or IT department to handle safelisting Mailchimp's servers.