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?

1.   Check the IP first in http://multirbl.valli.org/  and http://www.mxtoolbox.com. If found blacklisted, then de-list the IP.

E.g.:

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.