What is an email blocklist and how do I get off one?

An email blocklist (also called a blacklist or DNSBL) is a published list of mail servers and domains known for sending spam. Receiving providers like Gmail check incoming mail against these lists, so if your domain or your mail server’s IP address ends up on one, your legitimate email starts landing in spam or bouncing entirely:

• Why you get listed — a small business usually gets listed for one of three reasons: a compromised account sending spam, a misconfigured server, or shared hosting where another customer on the same IP misbehaved.
• How to check — run your domain and sending IP through a blacklist lookup like the one MXToolbox provides.
• How to get off — a two-step job: fix the underlying cause first — secure the account, correct the configuration — then submit a delisting request to each list that names you.

Delisting before you fix the cause just gets you relisted; clean up the source, then request removal.