If you received a message stating:
Currently, the sending IP address of xx.xxx.xxx.xxx - xxx.midphase.com is on a blacklist with xyz.
You can forward this message to support and they will work with the receiving email provider to remove the server from their blacklisting or provide you with alternative solutions.
You may also receive a bounce back email stating the email was blocked due to the sending MTA poor reputation. This is likely due to http://www.senderbase.org/index listing the IP as poor for various reasons. This commonly happens with shared IPs due to an account sending spam. This can also occur since many emails sent from a shared IP may identify themselves improperly due to our client's local computer or mail client settings and not as the name of the shared server that the IP RDNS is set up for. Unfortunately, in this case there usually isn't much that can be done as senderbase.org uses an automated system for tracking emails and we cannot force or continually monitor our clients email settings to follow proper email protocol.
Often times accounts are compromised due to insecure software and abused to send spam, resulting in the IP being black listed or having a poor reputation. The best way to prevent your email from being affected by other clients on the shared server is to purchase dedicated IP for your account.