Many Internet Service Providers block the default SMTP port, 25. If that is the case, you will want to use the alternate port 2525. How to do so depends on the type of account you have.
In order to use port 2525 on a site manager account, you must first have a dedicated IP address. If you have a shared IP address, please order a dedicated one here:
Once you have a dedicated IP address, you can install the port 2525 with these instructions: Sendmail Alternate Port
If you are on a cPanel Shared Hosting account, port 2525 is open by default.
If you are on a Cloud reseller account and port 2525 is not open by default, please do the following to open it:
Login to WHM and navigate to:
Scroll to the bottom and click Save.
If you are on a regular hosting plan (and not a reseller) then please contact technical support to enable port 2525 for you.