Why your emails are marked as SPAM?
It is very common for a mail IP address to end up on a public blacklist, especially on a shared server. It could be due to huge volume of emails coming from that server, or messages seems to have characteristics of spam in them. Each email SPAM filtering service check the reputation of incoming email, that means the reputation of incoming email server IP. If the IP address of the server is blacklisted, eventually the message will be marked as SPAM.
How do you overcome?
Always select the best email hosting services like Google Apps, Office 365 etc, where you have inbuilt technology so that the email server is not blacklisted. In fact, nobody blacklists Google or Microsoft!
Do not send your business marketing campaigns using your company email server. It is highly recommended to use newsletter services like Mailchimp, Salesforce to send out your marketing emails.
Always install an antivirus in the computers/devices where you use your email accounts, to avoid any malwares sending SPAM emails using your credentials.
Do not submit your email credentials on any SPAMMY links other than your webmail.Be aware of latest email SPAMMING techniques