Specific Email Fails and Defer Rate

[Yanzo]

Ahoy reader.

I have an issue regarding email fails which is it seems certain addresses cause my system to come back with errors which usually trips the defer rate error (note that these are valid email addresses).

I was wondering if there was any reason for this; the email extensions that usually come back with errors are AOL and Tesco which causes the system to shut down for an hour. This, as you'd expect, causes issues for us as we can't send any emails out for the next hour.
Is there any reason this is happening? Is there a problem with the route or the settings on their account or are these hosts just too strict with their filters?

Also, with the defer rate, when it says something like 5/5(20%) is the '5/5' bit regarding 5 out of 5 errors reached? Does the percentage refer to how many emails sent out in the past hour had errors?

Thanks.

[cPanelTristan]

Please post one of the failing messages return message error and the exim mainlog entry for it:

exigrep email@domain.com /var/log/exim_mainlog

Please replace the email address with the one that failed initially with some error and provide the exim log entry for what error it triggered. Without seeing that, we could not determine what might be happening.

[Yanzo]

Issue has been resolved. Our rDNS was missing some data which was giving us issues as now it's been corrected the errors have almost stopped completely.