Blacklist is a database of senders who have been reported as sending spam by the receivers. Email addresses from this list are believed to distribute spammy emails, so they are either blocked or flagged as spam in the recipient's folder.
High Volume of Mail: A server sending too many messages to subscribers in his email list can get flagged as spam.
Email Forwarding: When an IP relays spam to another amd when you forward this type of an email, it may seem like the spam is created by you.
Attachments: Sending too many attachments in your email marketing campaign may lead being blacklisted.
High Bounce Rate: This rate is a strong indicator for ISPs that a mailing list has not been legally obtained.
Including too Many Materials: Emails containing lots of graphic, image and animation materials trigger spam filters.
If your company is included in a blacklist, you can submit a request for removal to the website housing the blacklist. The fundamental here are to follow their instructions carefully and to obey their requests without demur.