When a lot of spam is sent from the server, website or server has been compromised, or server is used for illegal purposes, IP address of the affected may be blacklisted by one or more organisation dealing with abuse on the network.
When IP address is added to the blacklist, it may be impossible to send emails to some addresses.
If there have been a case of unauthorized access to your server after which you have problems with sending email, please use websites listed below to first check whether your IP address was blacklisted by any chance.
Automatic checker for most of the blacklists
http://whatismyipaddress.com/blacklist-check
Request form for removing IP address from Google's blacklist
https://support.google.com/mail/contact/bulk_send
Explanation for requesting removal of IP address from ALO's blacklist
http://postmaster.aol.com/
Please follow the instruction on the corresponding websites, as procedures vary depending on an organization. It usually takes from a few hours up to a few days to proceed removal request. However, even in case of it taking longer than usual, unfortunately we are unable to check the status of the request.