You may eventually encounter a problem sending email to certain domains because your IP address has been blacklisted.
It is your own and exclusive responsibility to keep your IP clean.
You are most likely to discover you have been blocked via bounce back emails informing you of this fact.
However, another common indication that you have been blacklisted is that emails to a certain domain are not being received, no matter what address on your server they are sent from or the email address within the domain in question they are being sent to.
If you are unsure if you have been blacklisted, there are a number of ways to check.
Check your IP first:
Debouncer or MX Toolbox’s Blacklist Checker and enter the IP address used to send your emails. These sites will check this IP against the blacklists of more than 100 major email providers and blacklist database.
When a bounce back email is related to a blacklisted IP, this bounce back should normally include the database used that reported the IP as being a SPAM emitter.