What really happened is that the emails sent to my customers (by their own customers) started to bounce,
because they were detected as spam.
I rarely ever get that problem. If I do, I advise the client that there is a reason why the sending mailserver is on a blacklist and that if that company/person really wants reliable email delivery they would need to contact their mail provider and sort out the reasons for the mailserver being listed.
Admittedly, depending upon where you are in the world, and depending upon the countries that your clientele operate, Zen may be blocking some mail that you consider legitimate. In the states I find it to be pretty darned useful. You may not. You can always look in your mail logs to determine the IP address of the mailserver being blocked and then you can add that IP address into the proper area in the Exim Configuration Manager to exempt that host from RBL checking so that their mail goes through.
M
