If you are using all your domains along with email accounts for all of them and it is all hosted on the single server/host then they will end up possibly getting flagged as mass emailing and then blocked.
We own a Rack Server located in a datacentre and
have several domains owned by different people on our server that we provide webspace and an email service for.
This worked great for a while and then started to get flagged and blocked and was given a 5xx error code stating we have been flagged as mass emailing.
We was like scratching our heads for a while thinking we are not mass emailing nothing, but in a way we are, if a server is hosting more than 1 mail service and is sending emails from each of those, its classed as mass emailing, even if its only 1 email a week per domain.
What we had to do is set up some SPF (Sender Policy Framework) domain records to say that the IP of the server is allowed to send emails for X domains.
Once that's set up correctly it all starts to work again.
Try reading this:
Hope that helps.
*** changes underlined ***
Edited by PaulKirby (Wed 27-Apr-16 03:06:49)