Each email provider uses their own unique criteria to determine how they filter and classify emails and filters can also be adjusted by individual users so inbox placement is not guaranteed. However there are a couple of key things you can do to greatly increase your odds of getting your messages into inboxes.
Most importantly, make sure that everybody you are emailing wants to receive email from you. Every time a recipient flags your message as spam, or you send to an email address that does not exist, it increases the likelihood that the email provider (and any email provider it shares information with) will classify your future messages as spam. We actively monitor spam complaints and do not allow sending unsolicited messages. Sending unsolicited emails may result in termination of your account.
Messages can be marked as spam due to many reasons, for example: bad content, bad list, poor back-end setup, spam filters.
You have control over the first two items: content and list.
It's your job to create content that is relevant to your readers and send emails only to contacts who expressed interest in receiving emails from you. By far the majority of spam complaints are a result of irrelevant content or content sent to people who didn't ask for it. (It's unavoidable that some people mark good content as spam because they forgot about their subscription, etc.)
We take care of the back-end setup such as managing SMTP servers, message code, unsubscribe mechanism., etc, however we recommend that you setup a custom subdomain to help with email delivery. In some cases you may also need to whitelist our sender IPs.