Due to no fault of your own your entire C-Block gets blacklisted and you can't get deliverability: IP's are issued by ARIN which is a nonprofit, member-based organization that administers IP addresses & ASNs in support of the operation and growth of the Internet. An IP address is your home. The C Block is your neighborhood. C Block IP is really only important for SEO if you find out your IP is in a C Block slum. A C-block is part of an IP-address. IP-addresses consist of 4 "blocks" of numbers, divided by dots. For example: 198.234. ...The C-block usually refers to the number of a webhost. Bad C Blocks: Sometimes a bad element can move into your neighborhood. If you're using a shared hosting provider you're essentially living in an apartment building. Other people live at that address. If every one of them except for you is a notorious porn site, it might not look so great. This could even happen on a larger level where an entire C Block has been used for some unsavory purpose. In this instance it's not just one building that's dilapidated, it's the entire neighborhood. Senders domains get blacklisted by association and into the junk/spam folder emails go.
Sales Type Emails: Emails that contain html, adjectives, bolding, underlining, italics, etc., automatically get assigned "points" by the Email service provider and SPAM Filters that use Bayesian filters. A Bayesian filter is a program that uses Bayesian logic, also called Bayesian analysis, to evaluate the header and content of an incoming e-mail message and determine the probability that it constitutes spam. Bayesian logic is an extension of the work of the 18th-century English mathematician Thomas Bayes. So all the common attention getting "salesmanship" used in your emails ends up sending emails to the Junk/SPAM folder.
The overwhelming majority of email senders do not have DKIM and SPF authentication properly set up.
There are many more reasons.
