Basically, what you want to do is check your domain against MULTIPLE blacklist checking tools to see if you need to take action. The ideal and perfect outcome is a bunch of green boxes and passing checkmarks. Never be extorted or exploited and pay for blacklist removal because these are fake services.
DNSBL, also called DNS black lists, are a resource used by email providers to keep their users’ inbox spam-free.
4 Email Blacklists you should care about…
You can also visit the Gmail help article and add / verify your domain so when you send email from OUTSIDE of gmail to a gmail hosted email address it won’t be blocked.
If you want to dig a little deeper, scan your actual IP address used by the email server… but you have to know that IP.