When yahoo generates a permanently deferred message, it will take approximately 4 days or 96 hours – sometimes even a week to get it start accepting the mails from the servers. Gmail and Hotmail, simply accept the mails, but it blackholes the email , means mails will simply be deleted, without any error message from the servers.

So the solution would be to identify the spam sources and remove them. However even after we remove the spam sources, customers wont start getting emails immediately. So a temporary work around is to change the IPs from which the emails are sent out.

By default emails are sent out from a mail server using it’s main shared IP. Here we are changing it. You can change it on per-account basis or for a entire server

The easiest method to see a full version of the files with proper syntax is to temporarily enable the following “automatic” option under WHM >> Main >> Service Configuration >> Exim Configuration Editor >> Standard Options >> Domains and IPs
“Automatically send outgoing mail from the account’s IP address instead of the main IP address.”

/etc/mailips will have the following format to be followed.

domain.com: xx.xx.xx.1
sub.domain.com: xx.xx.xx.2
addondomain.com: xx.xx.xx.2
*: xx.xx.xx.xx

If you want the entire server to use an alternate IP, use the last line alone which is the wild card entry.

*: new.ip.for.smtp

Sometimes you would want to set chattr +i /etc/mailips since, a cPanel update can clear the lines in it. And once you believe the main server is unblocked, remove the chattr. Please restart the exim server as well, after you made the changes in /etc/mailips

You would need to search for

remote_smtp:
driver = smtp

and make sure that a line similar to below exists, which does literal search for the IPs in /etc/mailips. If /etc/mailips lines are not there, you can simply add interface=xx.xx.xx.xx entries.

interface = ${if exists {/etc/mailips}{${lookup{$sender_address_domain}lsearch*{/etc/mailips}{$value}{$
helo_data = ${if exists {/etc/mailhelo}{${lookup{$sender_address_domain}lsearch*{/etc/mailhelo}{$value$

Ref : http://forums.cpanel.net/f43/formats-etc-mailhelo-mailips-etc-111977.html for more details and few more formats.