Email alerts sending to defunct address

Greetings,

I have a Urbackup server 2.5.36. The server has 24 clients. A while back we changed the destination email for alerts. The issue I am having is that four of the clients continues to send email to the old address. I have confirmed on the server and on the client settings file, that the address is correct.

Any ideas would be appreciated.

Thanks,
Sean.