Free tool
Blacklist (RBL) Checker
Check whether a mail server IP address is listed on the major spam blacklists, which is the usual reason outgoing email is rejected or silently dropped.
What to do if you are listed
Find the cause before requesting removal. A listing is a symptom, and almost always means one of three things: a device on your network is infected and sending spam, a mailbox password was stolen and is being used to relay, or your server is an open relay. Delisting without fixing the cause gets you listed again within days, and repeat listings are harder to clear.
Not all lists carry the same weight. Spamhaus ZEN is the one that actually stops mail, because the large providers consult it directly. Some of the more aggressive lists will list an entire hosting range because of one neighbour, and most receiving servers ignore them, so a listing there alone is rarely the reason your mail is bouncing.
Check your own IP before blaming the recipient. If your server is clean here, the problem is usually missing SPF, DKIM or DMARC records instead, which you can check with our email records tool. Those three are behind far more delivery failures than blacklists are.