Some of our customers are complaining that they can't receive any emails from some of their clients. So I proceed to disabled the protocol checks for unqualified (non fqdn) senders and domains in Zimbra configuration.
Ideally, Zimbra should check the sender's domain and verify if this domain MX record match with the email server that it is coming from. But I suspect that Zimbra only check if the sender's domain match with the email server that it is coming from instead.
If this is the case, then we will encounter many senders getting rejected because most senders with their own domain but hosted their emails with their ISP will have their MX records pointing to their ISP MX instead of their own domain MX.
Does anyone has any idea which method does Zimbra used to verify fqdn (Fully qualified domain name)?