Most of the time IP is blacklisted due to its reputation, for the time duration during which the IP has been a culprit of spamming , mass mailing or malicious activity. If IP reputation goes below a particular level, then it is also blacklisted by all major ISP like: Yahoo, Microsoft (outlook), Hotmail, Gmail, Rediff, AOL etc.
How to check IP blacklisting?
2.) Mostly IPs are blacklisted in major authorities, like Barracuda, Spamhaus, Sorbs, Lashback and mailspike.
Check IP in the above websites and then raise de-list request through the link provided in step1. Be careful that the issue should be resolved first and then only the De-list request should be raised.